cat: avoid splice for small files

[oech3]

Closes #10835
@FidelSch Would you bench? My hyperfile is not stable.

[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)
Skipping an intermittent issue tests/tail/inotify-dir-recreate (passes in this run but fails in the 'main' branch)
Skipping an intermittent issue tests/tail/symlink (passes in this run but fails in the 'main' branch)
Note: The gnu test tests/tail/tail-n0f is now being skipped but was previously passing.
Congrats! The gnu test tests/dd/no-allocate is now passing!

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/tail/follow-name (fails in this run but passes in the 'main' branch)
Congrats! The gnu test tests/misc/io-errors is no longer failing!
Note: The gnu test tests/env/env-signal-handler was skipped on 'main' but is now failing.

[oech3]

CodSpeed has no change. @FidelSch Are you OK to close this and your PR?

[oech3]

But it might needed to avoid pipe and fcntl for small files.

[codspeed-hq]

Merging this PR will not alter performance

✅ 2 untouched benchmarks
⏩ 348 skipped benchmarks¹

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_{Comparing oech3:cat-small-file (b6bb1bf) with main (236e220)}

Footnotes

1. 348 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. ↩

[oech3]

I guess overhead is pipe() instead of splice().