Implement statfs with synthetic values

[c-hagem]

Description of change

This PR adds support for calling statfs on virtual file system created using mountpoint.
Some applications depend on the filesystem reporting non-zero available space; currently mountpoint reports 0 as number of available blocks, which can cause these applications to not work as expected.

This PR (building on #871) implements statfs with synthetic values (4611686018427387904 free blocks).
For example, the DF output now is:

mountpoint-s3  4611686018427387904        0 4611686018427387904   0% /local/home/chagem/mnt/bucket

Thus, checks for available space should no longer fail.

Relevant issues: #710.

Does this change impact existing behavior?

This change impacts existing behaviour, as Mountpoint will report non-zero value for total blocks, free blocks, free inodes and maximum file name length.

Does this change need a changelog entry?

Yes, addressed.

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[dannycjones]

Nice! Awesome to see this working.

There are a few comments inline.

Can you add some tests for this, just to verify these values make it out of the file system? I'd recommend creating a new file in mountpoint-s3/tests/fuse_tests/ named statfs_test.rs, and write some tests that test end-to-end. statvfs isn't actually in the Rust standard library, but we have some example of using a crate named nix for querying this information.

mountpoint-s3/mountpoint-s3/src/data_cache/disk_data_cache.rs

Lines 308 to 321 in 9206ed4

	CacheLimit::AvailableSpace { min_ratio } => {
	let stats = match nix::sys::statvfs::statvfs(&self.cache_directory) {
	Ok(stats) if stats.blocks() == 0 => {
	warn!("unable to determine available space (0 blocks reported)");
	return false;
	}
	Ok(stats) => stats,
	Err(error) => {
	warn!(?error, "unable to determine available space");
	return false;
	}
	};
	(stats.blocks_free() as f64) < min_ratio * (stats.blocks() as f64)
	}

[dannycjones]

@c-hagem The rebase appears to have included changes unrelated to this PR, can you fix?

[dannycjones]

LGTM, ship it! 🎉

[dannycjones]

Released in Mountpoint v1.12.0! 🚀