io_uring: refactor io_resubmit_prep()

It's awkward to pass return a value into a function for it to return it
back. Check it at the caller site and clean up io_resubmit_prep() a bit.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>