io_uring: merge CQ and poll waitqueues

->cq_wait and ->poll_wait and waken up in the same manner, use a single
waitqueue for both of them. CQ waiters are queued exclusively, so wake
up should first go over all pollers and that's what we need.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/00fe603e50000365774cf8435ef5fe03f049c1c9.1633373302.git.asml.silence@gmail.com


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