mkfifo: fix build where mode_t is not u32

[pkubaj]

rustix::fs::Mode is a bitflags over rustix::fs::RawMode, which aliases libc::mode_t. That is u32 on Linux but u16 on the BSDs, so handing the u32 mode straight to Mode::from_bits_truncate() fails to compile there:

error[E0308]: mismatched types
   --> src/uu/mkfifo/src/mkfifo.rs:146:50
    |
146 |     mkfifoat(CWD, path, Mode::from_bits_truncate(mode))
    |                         ------------------------ ^^^^ expected `u16`, found `u32`

Cast to RawMode first, which is a no-op on Linux and matches what mkdir already does in src/uu/mkdir/src/mkdir.rs.

[codspeed-hq]

Merging this PR will degrade performance by 3.43%

⚠️ Different runtime environments detected

Some benchmarks with significant performance changes were compared across different runtime environments,
which may affect the accuracy of the results.

Open the report in CodSpeed to investigate

❌ 2 regressed benchmarks
✅ 344 untouched benchmarks
⏩ 47 skipped benchmarks¹

Warning

Please fix the performance issues or acknowledge them on CodSpeed.

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
❌	Simulation	cksum_multiple_files	60.6 ms	63 ms	-3.8%
❌	Simulation	du_max_depth_balanced_tree[(6, 4, 10)]	60.1 ms	62 ms	-3.05%

Tip

Investigate this regression by commenting @codspeedbot fix this regression on this PR, or directly use the CodSpeed MCP with your agent.

---

_{Comparing pkubaj:patch-1 (8020cb1) with main (667a82b)}

Footnotes

1. 47 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩

[sylvestre]

on which bsd? thanks

[pkubaj]

FreeBSD.

[github-actions]

GNU testsuite comparison:

GNU test failed: tests/misc/sync. tests/misc/sync is passing on 'main'. Maybe you have to rebase?
Skip an intermittent issue tests/cut/bounded-memory (fails in this run but passes in the 'main' branch)
Skip an intermittent issue tests/date/date-locale-hour (fails in this run but passes in the 'main' branch)
Skip an intermittent issue tests/timeout/timeout-group (fails in this run but passes in the 'main' branch)
Note: The gnu test tests/env/env-signal-handler was skipped on 'main' but is now failing.