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.