splice: add wakeup_pipe_readers()

Add and use wakeup_pipe_readers() to consolidate duplicated codes.

Signed-off-by: Namhyung Kim <namhyung@gmail.com>
Cc: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Jens Axboe <jaxboe@fusionio.com>