Fix warnings on stable Rust

[unexge]

Hey! We currently maintain a fork of fuser for own use, and we use stable Rust toolchain (v1.82 as of today). As Rust got better on detecting unused code since v1.74 (the version used in fuser), we get some warning during compilation.

cargo +stable check

warning: unused import: `RequestError`
  --> src/ll/mod.rs:14:64
   |
14 |     AnyRequest, FileHandle, INodeNo, Lock, Operation, Request, RequestError, RequestId, Version,
   |                                                                ^^^^^^^^^^^^
   |
   = note: `#[warn(unused_imports)]` on by default

warning: field `0` is never read
    --> src/ll/request.rs:1888:14
     |
1888 |     ReadLink(ReadLink<'a>),
     |     -------- ^^^^^^^^^^^^
     |     |
     |     field in this variant
     |
     = note: `Operation` has a derived impl for the trait `Debug`, but this is intentionally ignored during dead code analysis
     = note: `#[warn(dead_code)]` on by default
help: consider changing the field to be of unit type to suppress this warning while preserving the field numbering, or remove the field
     |
1888 |     ReadLink(()),
     |              ~~

warning: field `0` is never read
    --> src/ll/request.rs:1899:12
     |
1899 |     StatFs(StatFs<'a>),
     |     ------ ^^^^^^^^^^
     |     |
     |     field in this variant
     |
     = note: `Operation` has a derived impl for the trait `Debug`, but this is intentionally ignored during dead code analysis
help: consider changing the field to be of unit type to suppress this warning while preserving the field numbering, or remove the field
     |
1899 |     StatFs(()),
     |            ~~

warning: field `0` is never read
   --> src/ll/request.rs:877:14
    |
877 |         Size(NonZeroU32),
    |         ---- ^^^^^^^^^^
    |         |
    |         field in this variant
    |
    = note: `GetXAttrSizeEnum` has a derived impl for the trait `Debug`, but this is intentionally ignored during dead code analysis
help: consider changing the field to be of unit type to suppress this warning while preserving the field numbering, or remove the field
    |
877 |         Size(()),
    |              ~~

warning: struct `CopyFileRangeFile` is never constructed
    --> src/ll/request.rs:1521:16
     |
1521 |     pub struct CopyFileRangeFile {
     |                ^^^^^^^^^^^^^^^^^
     |
     = note: `CopyFileRangeFile` has derived impls for the traits `Clone` and `Debug`, but these are intentionally ignored during dead code analysis

warning: `fuser` (lib) generated 5 warnings (run `cargo fix --lib -p fuser` to apply 1 suggestion)
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.03s

This PR fixes these warnings. I also verified each feature flag using cargo hack to ensure each of them still compiles without any warnings.

cargo +stable hack check --each-feature --no-dev-deps

info: --no-dev-deps modifies real `Cargo.toml` while cargo-hack is running and restores it when finished
info: running `cargo check --all-features` on fuser (1/29)
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.05s

info: running `cargo check --no-default-features` on fuser (2/29)
   Compiling fuser v0.15.0 (/workplace/burakvar/fuser)
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.34s

info: running `cargo check --no-default-features --features abi-7-10` on fuser (3/29)
   Compiling fuser v0.15.0 (/workplace/burakvar/fuser)
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.38s

info: running `cargo check --no-default-features --features abi-7-11` on fuser (4/29)
   Compiling fuser v0.15.0 (/workplace/burakvar/fuser)
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.37s

info: running `cargo check --no-default-features --features abi-7-12` on fuser (5/29)
   Compiling fuser v0.15.0 (/workplace/burakvar/fuser)
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.41s

info: running `cargo check --no-default-features --features abi-7-13` on fuser (6/29)
   Compiling fuser v0.15.0 (/workplace/burakvar/fuser)
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.38s

info: running `cargo check --no-default-features --features abi-7-14` on fuser (7/29)
   Compiling fuser v0.15.0 (/workplace/burakvar/fuser)
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.44s

info: running `cargo check --no-default-features --features abi-7-15` on fuser (8/29)
    Blocking waiting for file lock on build directory
   Compiling fuser v0.15.0 (/workplace/burakvar/fuser)
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.95s

info: running `cargo check --no-default-features --features abi-7-16` on fuser (9/29)
   Compiling fuser v0.15.0 (/workplace/burakvar/fuser)
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.41s

info: running `cargo check --no-default-features --features abi-7-17` on fuser (10/29)
   Compiling fuser v0.15.0 (/workplace/burakvar/fuser)
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.38s

info: running `cargo check --no-default-features --features abi-7-18` on fuser (11/29)
   Compiling fuser v0.15.0 (/workplace/burakvar/fuser)
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.39s

info: running `cargo check --no-default-features --features abi-7-19` on fuser (12/29)
   Compiling fuser v0.15.0 (/workplace/burakvar/fuser)
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.38s

info: running `cargo check --no-default-features --features abi-7-20` on fuser (13/29)
   Compiling fuser v0.15.0 (/workplace/burakvar/fuser)
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.39s

info: running `cargo check --no-default-features --features abi-7-21` on fuser (14/29)
   Compiling fuser v0.15.0 (/workplace/burakvar/fuser)
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.39s

info: running `cargo check --no-default-features --features abi-7-22` on fuser (15/29)
   Compiling fuser v0.15.0 (/workplace/burakvar/fuser)
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.39s

info: running `cargo check --no-default-features --features abi-7-23` on fuser (16/29)
   Compiling fuser v0.15.0 (/workplace/burakvar/fuser)
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.39s

info: running `cargo check --no-default-features --features abi-7-24` on fuser (17/29)
   Compiling fuser v0.15.0 (/workplace/burakvar/fuser)
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.39s

info: running `cargo check --no-default-features --features abi-7-25` on fuser (18/29)
   Compiling fuser v0.15.0 (/workplace/burakvar/fuser)
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.39s

info: running `cargo check --no-default-features --features abi-7-26` on fuser (19/29)
   Compiling fuser v0.15.0 (/workplace/burakvar/fuser)
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.39s

info: running `cargo check --no-default-features --features abi-7-27` on fuser (20/29)
   Compiling fuser v0.15.0 (/workplace/burakvar/fuser)
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.39s

info: running `cargo check --no-default-features --features abi-7-28` on fuser (21/29)
   Compiling fuser v0.15.0 (/workplace/burakvar/fuser)
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.39s

info: running `cargo check --no-default-features --features abi-7-29` on fuser (22/29)
   Compiling fuser v0.15.0 (/workplace/burakvar/fuser)
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.40s

info: running `cargo check --no-default-features --features abi-7-30` on fuser (23/29)
   Compiling fuser v0.15.0 (/workplace/burakvar/fuser)
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.41s

info: running `cargo check --no-default-features --features abi-7-31` on fuser (24/29)
   Compiling fuser v0.15.0 (/workplace/burakvar/fuser)
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.39s

info: running `cargo check --no-default-features --features abi-7-9` on fuser (25/29)
   Compiling fuser v0.15.0 (/workplace/burakvar/fuser)
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.32s

info: running `cargo check --no-default-features --features default` on fuser (26/29)
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.02s

info: running `cargo check --no-default-features --features libfuse` on fuser (27/29)
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.02s

info: running `cargo check --no-default-features --features macfuse-4-compat` on fuser (28/29)
   Compiling fuser v0.15.0 (/workplace/burakvar/fuser)
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.32s

info: running `cargo check --no-default-features --features serializable` on fuser (29/29)
   Compiling fuser v0.15.0 (/workplace/burakvar/fuser)
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.35s