Restore statx request-mask filtering

[a-shannon]

Summary

PR #379 added a forward-compatibility guard that masks unknown statx
request bits before invoking the kernel. After #787 enabled bitflags'
externally-defined-flags support, StatxFlags::all() became u32::MAX, so
mask & StatxFlags::all() became an identity operation and the guard stopped
filtering unknown bits.

This change:

• introduces a private StatxRequestFlags containing the 14 currently named
  elementary request bits;
• sanitizes requests before either backend or the compatibility path;
• continues rejecting STATX__RESERVED with EINVAL;
• keeps the public StatxFlags external-flags behavior unchanged; and
• adds a causal test proving that the sanitized set equals the named flags,
  not StatxFlags::all().

This restores the future-kernel safety boundary intended by #379. It does not
claim a vulnerability on current kernels.

Testing

Focused local validation recorded:

• linux_raw: causal unit 1/1 and statx integration 2/2;
• use-libc: causal unit 1/1 and statx integration 2/2;
• focused library and fs test-target Clippy with -D warnings for both
  backends;
• git diff --check; and
• independent source review of the exact diff with no findings.

This does not claim a repository-wide Clippy, MSRV, or CI pass.

Relation to #1467

This is complementary to #1467, not a replacement for it. #1467 adds
MNT_ID_UNIQUE, SUBVOL, WRITE_ATOMIC, and the write-atomic attribute.
Whichever PR lands second should extend the private request allowlist with the
newly named request bits during rebase.