relay: use splice_to_pipe() instead of open-coding the pipe loop

It cleans up the relay splice implementation a lot, and gets rid of
a lot of internal pipe knowledge that should not be in there.

Plus fixes for padding and partial first page (and lots more) from
Tom Zanussi.

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>

diff --cc kernel/relay.c
Simple merge