io_uring: clean io_prep_rw()

We already store req->file in a variable in io_prep_rw(), just use it
instead of a couple of left references to kicob->ki_filp.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/2f5889fc7ab670daefd5ccaedd99416d8355f0ad.1634314022.git.asml.silence@gmail.com


Signed-off-by: Jens Axboe <axboe@kernel.dk>