cat: improve splice fast-path

[oech3]

Fixes #11516

$ yes-gf421d0112 | cat-p | pv >/dev/null
^C.4GiB 0:00:04 [23.1GiB/s]
$ yes-gf421d0112 | cat | pv >/dev/null
^C.6GiB 0:00:05 [6.33GiB/s]

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/cut/bounded-memory (fails in this run but passes in the 'main' branch)
Skipping an intermittent issue tests/date/date-locale-hour (passes in this run but fails in the 'main' branch)
Note: The gnu test tests/basenc/bounded-memory is now being skipped but was previously passing.

[codspeed-hq]

Merging this PR will degrade performance by 4.98%

❌ 1 regressed benchmark
✅ 303 untouched benchmarks
⏩ 46 skipped benchmarks¹

⚠️ Please fix the performance issues or acknowledge them on CodSpeed.

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
❌	Simulation	cp_large_file[16]	264.3 µs	278.2 µs	-4.98%

---

_{Comparing oech3:splice-pipe (ac58184) with main (5f9f735)²}

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. ↩

2. No successful run was found on main (236e220) during the generation of this report, so 5f9f735 was used instead as the comparison base. There might be some changes unrelated to this pull request in this report. ↩

[github-actions]

GNU testsuite comparison:

Note: The gnu test tests/pr/bounded-memory is now being skipped but was previously passing.
Congrats! The gnu test tests/tail/pipe-f is now passing!

[oech3]

truncate -s 9EB huge; cat-p huge | pv>/dev/null is 3~10GiB/s faster than zero-copy yes now...

$ taskset -c 1 cat-p huge |taskset -c 2 pv>/dev/null
^C39GiB 0:00:06 [40.0GiB/s]
$ taskset -c 1 cat huge |taskset -c 2 pv>/dev/null
^C92GiB 0:00:15 [20.7GiB/s]

[oech3]

I have no idea how to achieve yes which has same perf with cat sparse-huge.
I considered that tee()ing 1 MiB pipe with references of y is same with splice()ing a zero page, but it has simllar perf with real 1 MiB y.

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/pr/bounded-memory (fails in this run but passes in the 'main' branch)
Skip an intermittent issue tests/tty/tty-eof (fails in this run but passes in the 'main' branch)
Skipping an intermittent issue tests/cut/bounded-memory (passes in this run but fails in the 'main' branch)
Note: The gnu test tests/csplit/csplit-heap is now being skipped but was previously passing.
Note: The gnu test tests/tail/tail-n0f is now being skipped but was previously passing.
Congrats! The gnu test tests/basenc/bounded-memory is now passing!
Congrats! The gnu test tests/dd/no-allocate is now passing!

[oech3]

Is this OK to merge?