recvmsg_uninit does not work with MSG_TRUNC

[andylizi]

recvfrom(2) has a MSG_TRUNC flag that makes it "return the real length of the packet or datagram, even when it was longer than the passed buffer."

This leads to panic since rustix::net::recvmsg_uninit tries to split the return buffer according to the returned length.

// Pass in a really small buffer
let (buf, _, _) = rustix::net::recvfrom_uninit(&socket, &mut buf, RecvFlags::TRUNC)?;

thread 'main' panicked at ~/.cargo/git/checkouts/rustix-8fafe793515fde97/9b432db/src/buffer.rs:18:30:
mid > len
stack backtrace:
   2: core::slice::<impl [T]>::split_at_mut
             at /rustc/0d634185dfddefe09047881175f35c65d68dcff1/library/core/src/slice/mod.rs:1906:21
   3: rustix::buffer::split_init
             at ~/.cargo/git/checkouts/rustix-8fafe793515fde97/9b432db/src/buffer.rs:18:26
   4: rustix::net::send_recv::recvfrom_uninit
             at ~/.cargo/git/checkouts/rustix-8fafe793515fde97/9b432db/src/net/send_recv/mod.rs:169:35

rustix::net::recvfrom doesn't suffer from this issue since it just returns the length directly. But it also doesn't support uninit buffers.