pipes.rs: make splice_exact robust against splice failure

[oech3]

This error is very rare (and maybe impossible to add regression test).
But if N-th splice of splice_exact failed, the length used by write fallback is wrong. Thus, it should return length failed to splice.

Better replacement for #12315

[oech3]

Impossible to include fallback to the fn... I'll dedup the code at different PR.

[github-actions]

GNU testsuite comparison:

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.
Note: The gnu test tests/tail/tail-n0f is now being skipped but was previously passing.
Congrats! The gnu test tests/printf/printf-surprise is now passing!

[oech3]

I found better way. Closing.