io_uring: pass only !null to io_req_find_next()

Make io_req_find_next() and io_req_link_next() to accept only non-null
nxt, and handle it in callers.

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