cat: splice fast-path is slower than GNU yes

[oech3]

Needs GNU yes supporting zero-copy (no stable commit)

$ gnu58d88d243/yes-gf421d0112 | cat | pv>/dev/null
^C.3GiB 0:00:06 [6.37GiB/s]
$ gnu58d88d243/yes-gf421d0112 | pv>/dev/null
^C.7GiB 0:00:02 [24.9GiB/s]

I guess we are not extending pipe size to 1 MiB at splice path.