io_uring: improve sqpoll event/state handling

As sqd->state changes rarely, don't check every event one by one but
look them all at once. Add a helper function. Also don't go into event
waiting sleeping with STOP flag set.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/645025f95c7eeec97f88ff497785f4f1d6f3966f.1621201931.git.asml.silence@gmail.com


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