block: fix checking return value of blk_mq_init_queue

Check IS_ERR_OR_NULL(return value) instead of just return value.

Signed-off-by: Ming Lei <ming.lei@canonical.com>

Reduced to IS_ERR() by me, we never return NULL.
Signed-off-by: Jens Axboe <axboe@fb.com>

drivers/block/null_blk.c

@@ -530,7 +530,7 @@ static int null_add_dev(void)
 			goto out_cleanup_queues;
 
 		nullb->q = blk_mq_init_queue(&nullb->tag_set);
-		if (!nullb->q) {
+		if (IS_ERR(nullb->q)) {
 			rv = -ENOMEM;
 			goto out_cleanup_tags;
 		}