io_uring: compare cached_cq_tail with cq.head in_io_uring_poll

After 75b28aff("io_uring: allocate the two rings together"), we compare
sq.head with cached_cq_tail to determine does there any cq invalid.
Actually, we should use cq.head.

Fixes: 75b28aff ("io_uring: allocate the two rings together")
Signed-off-by: yangerkun <yangerkun@huawei.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

fs/io_uring.c

@@ -3455,7 +3455,7 @@ static __poll_t io_uring_poll(struct file *file, poll_table *wait)
 	if (READ_ONCE(ctx->rings->sq.tail) - ctx->cached_sq_head !=
 	    ctx->rings->sq_ring_entries)
 		mask |= EPOLLOUT | EPOLLWRNORM;
-	if (READ_ONCE(ctx->rings->sq.head) != ctx->cached_cq_tail)
+	if (READ_ONCE(ctx->rings->cq.head) != ctx->cached_cq_tail)
 		mask |= EPOLLIN | EPOLLRDNORM;
 
 	return mask;