pipes.rs: remove fcntl overhead from splice_unbounded

[oech3]

Any splice error is discarded. So no benefit for conversion.
Closes #12419 .
Fix one of a bool issue seen #12365 .
Also avoid fcntl for small input.

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/date/date-locale-hour (fails in this run but passes in the 'main' branch)
Skip an intermittent issue tests/misc/io-errors (fails in this run but passes in the 'main' branch)
Skip an intermittent issue tests/rm/isatty (fails in this run but passes in the 'main' branch)
Congrats! The gnu test tests/cut/cut-huge-range is now passing!

[codspeed-hq]

Merging this PR will not alter performance

✅ 319 untouched benchmarks
⏩ 46 skipped benchmarks¹

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_{Comparing oech3:useless-conversion (3790a76) with main (bd908f0)}

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. ↩