sum: take metadata from fd instead of path

[oech3]

Closes #12947

[codspeed-hq]

Merging this PR will not alter performance

✅ 323 untouched benchmarks
⏩ 46 skipped benchmarks¹

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_{Comparing oech3:sum-toctou (31ce44d) with main (ecfaf5a)}

Footnotes

1. 46 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. ↩

[github-actions]

GNU testsuite comparison:

Skipping an intermittent issue tests/date/date-locale-hour (passes in this run but fails in the 'main' branch)
Skipping an intermittent issue tests/pr/bounded-memory (passes in this run but fails in the 'main' branch)
Congrats! The gnu test tests/cp/sparse-2 is now passing!
Congrats! The gnu test tests/tail/pipe-f is now passing!
Note: The gnu test tests/misc/write-errors was skipped on 'main' but is now failing.