fsext.rs: use type inference fsid_t / __fsid_t

[Ecordonnier]

Commit 2a0d58d (part of #3396 which contains a description of the changes) changed this line from libc::fsid_t to nix::sys::statfs::fsid_t. The pull-request description at #3396 indicates that this was done in order to fix the android build, and indeed using a cast to nix::sys::statfs::fsid_t takes advantage of the definition of nix::sys::statfs::fsid_t which abstracts away the different name on Android:

/// Identifies a mounted file system
#[cfg(target_os = "android")]
pub type fsid_t = libc::__fsid_t;
/// Identifies a mounted file system
#[cfg(not(target_os = "android"))]
pub type fsid_t = libc::fsid_t;

This cast works as long as the libc version used by nix is the same than the libc version used by coreutils.

This cast becomes invalid when using a local libc version for local debugging, and changing Cargo.toml to point to it:

-libc = "0.2.153"
+libc = { path = "../path/to/libc" }

The cast becomes invalid because self.f_fsid is of type libc::fsid_t (local version of libc), whereas nix::sys::statfs::fsid_t still uses the libc version downloaded by cargo from crates.io in this case.

I was getting this error:

coreutils$ cargo build
   Compiling libc v0.2.171 (/home/ecordonnier/dev/libc)
   Compiling uucore v0.0.30 (/home/ecordonnier/dev/coreutils/src/uucore)
error[E0606]: casting `&libc::fsid_t` as `*const nix::libc::fsid_t` is invalid
   --> src/uucore/src/lib/features/fsext.rs:816:25
    |
816 |             unsafe { &*(&self.f_fsid as *const nix::sys::statfs::fsid_t as *const [u32; 2]) };
    |                         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

For more information about this error, try `rustc --explain E0606`.
error: could not compile `uucore` (lib) due to 1 previous error

Let's rather use type inference to deal with libc::fsid_t vs libc::__fsid_t.

[github-actions]

GNU testsuite comparison:

Skipping an intermittent issue tests/misc/stdbuf (passes in this run but fails in the 'main' branch)

[cakebaker]

Thanks!