chmod: use the libc fchmodat symbol for recursive mode changes

[sylvestre]

Fixes #14028

[Copilot]

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[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/tail/pid-pipe (fails in this run but passes in the 'main' branch)
Skip an intermittent issue tests/tail/tail-n0f (fails in this run but passes in the 'main' branch)
Skipping an intermittent issue tests/cut/bounded-memory (passes in this run but fails in the 'main' branch)
Congrats! The gnu test tests/seq/seq-epipe is now passing!

[codspeed-hq]

Merging this PR will improve performance by 3.22%

⚠️ 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

⚡ 4 improved benchmarks
✅ 353 untouched benchmarks
⏩ 50 skipped benchmarks¹

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
⚡	Simulation	du_summarize_balanced_tree[(5, 4, 10)]	16.5 ms	15.9 ms	+3.42%
⚡	Simulation	cksum_sysv	68.4 ms	66.2 ms	+3.32%
⚡	Simulation	cksum_crc32b	39.7 ms	38.5 ms	+3.11%
⚡	Simulation	cksum_multiple_files	61.5 ms	59.7 ms	+3.01%

Tip

Curious why this is faster? Comment @codspeedbot explain why this is faster on this PR, or directly use the CodSpeed MCP with your agent.

---

_{Comparing sylvestre:preload (e3e5876) with main (ebb9ab1)}

Footnotes

1. 50 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. ↩

[Copilot]

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[sylvestre]

@nadzyah
@julian-klode
wdyt ? :)
thanks

[sylvestre]

@codspeedbot explain why this is faster

[nadzyah]

@sylvestre hey! Sorry, I didn't review it this quickly as I wanted to test the change

I've verified that it fixes the issue. I reverted safe_traversal.rs to the base commit and ran the new check. It fails (fakeroot reports '775', expected '2751') and passes with your change, so the regression test really does catch the bug. I also checked a deeper tree under fakeroot (every level is recorded, setgid included) and compared against GNU chmod for -R 755, -R 2751, -R 1777, -R 4755, -R u+s,g+s,o+t, -R a+X and -R -L/-H/-P. All modes match!

Thank you for the fix!

[sylvestre]

@nadzyah np and thanks for testing !
btw maybe fakeroot should be less used ;)