io_uring: waitqueue-less cq waiting

With DEFER_TASKRUN only ctx->submitter_task might be waiting for CQEs,
we can use this to optimise io_cqring_wait(). Replace ->cq_wait
waitqueue with waking the task directly.

It works but misses an important optimisation covered by the following
patch, so this patch without follow ups might hurt performance.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/103d174d35d919d4cb0922d8a9c93a8f0c35f74a.1673274244.git.asml.silence@gmail.com


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