fs/splice: Pull buf->ops->confirm() from splice_from_pipe actors

fs/splice: Pull buf->ops->confirm() from splice_from_pipe actors

This patch pulls calls to buf->ops->confirm() from all actors passed
(also indirectly) to splice_from_pipe_feed().

Is avoiding the call to buf->ops->confirm() while splice()ing to
/dev/null is an intentional optimization? No other user does that
and this will remove this special case.

Against current linux.git 4179f42bb42d394a3045d6bc3151c626281bcb5f.

Signed-off-by: Michał Mirosław <mirq-linux@rere.qmqm.pl>
Signed-off-by: Jens Axboe <jaxboe@fusionio.com>