io_uring: consolidate *_check_overflow accounting

Add a helper to mark ctx->{cq,sq}_check_overflow to get rid of
duplicates, and it's clearer to check cq_overflow_list directly anyway.

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