io_uring: refactor io_cqring_wait

It's easy to make a mistake in io_cqring_wait() because for all
break/continue clauses we need to watch for prepare/finish_wait to be
used correctly. Extract all those into a new helper
io_cqring_wait_schedule(), and transforming the loop into simple series
of func calls: prepare(); check_and_schedule(); finish();

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