du: count reflink disk usage only once

[aaron-ang]

Close #3906.

Dedupe reflink bytes only when --apparent-size or --bytes flag is set, based on man page.

Implementation

• Keep track of shared extents while walking extent mapping using fiemap ioctl: https://www.kernel.org/doc/Documentation/filesystems/fiemap.txt.
• Only increment bytes if extent has not been seen.

[github-actions]

GNU testsuite comparison:

Skipping an intermittent issue tests/timeout/timeout (passes in this run but fails in the 'main' branch)

[github-actions]

GNU testsuite comparison:

GNU test failed: tests/shuf/shuf-reservoir. tests/shuf/shuf-reservoir is passing on 'main'. Maybe you have to rebase?
GNU test failed: tests/sort/sort-stale-thread-mem. tests/sort/sort-stale-thread-mem is passing on 'main'. Maybe you have to rebase?
Skipping an intermittent issue tests/timeout/timeout (passes in this run but fails in the 'main' branch)

[github-actions]

GNU testsuite comparison:

Skipping an intermittent issue tests/timeout/timeout (passes in this run but fails in the 'main' branch)
Skipping an intermittent issue tests/tty/tty-eof (passes in this run but fails in the 'main' branch)

[codspeed-hq]

Merging this PR will improve performance by 3.51%

Summary

⚡ 1 improved benchmark
✅ 139 untouched benchmarks
⏩ 37 skipped benchmarks¹

Performance Changes

	Benchmark	BASE	HEAD	Efficiency
⚡	tsort_input_parsing_heavy[5000]	73.7 ms	71.2 ms	+3.51%

---

_{Comparing aaron-ang:du-reflink-count (5bc43b7) with main (20a5c3a)}

Footnotes

1. 37 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. ↩