[Virtio-fs] [virtiofsd-rs] MR merged: Avoid using libc::statx()

[virtiofs-bot at sinrega.org]

This patch should resolve issue #12 (in a different manner than !22, as suggested by Connor).

`libc::statx()` resolves to the system libc's `statx()` (unsurprisingly), which may or may not be present.  For example, in glibc before 2.28, it was not present (regardless of whether the kernel supports the statx system call or not), leading to 

The libc function is just a wrapper around the syscall anyway, so we might as well implement the wrapper ourselves.  This has the advantage of only relying on `libc::syscall()` (which, on Linux, is basically always there) and `libc::SYS_statx` (which is provided by the `libc` crate, so does not depend on the system libc).

This way, we do not need to check whether the system libc provides the wrapper, and our `statx()` really works whenever the kernel supports it.
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https://gitlab.com/virtio-fs/virtiofsd-rs/-/merge_requests/25