io_uring: refactor io_req_find_next

Move the fast path from io_req_find_next() into callers. It prepares us
for further changes.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/10bd0e564472dde0c7f8d90ae317d05356cd565a.1647897811.git.asml.silence@gmail.com


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