io_uring: replace find_next() out param with ret

Generally, it's better to return a value directly than having out
parameter. It's cleaner and saves from some kinds of ugly bugs.
May also be faster.

Return next request from io_req_find_next() and friends directly
instead of passing out parameter.

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