null_blk: Fix null pointer dereference on nullb->disk on blk_cleanup_disk call

The error handling on a nullb->disk allocation currently jumps to
out_cleanup_disk that calls blk_cleanup_disk with a null pointer causing
a null pointer dereference issue. Fix this by jumping to out_cleanup_tags
instead.

Addresses-Coverity: ("Dereference after null check")
Fixes: 132226b3 ("null_blk: convert to blk_alloc_disk/blk_cleanup_disk")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Reviewed-by: Chaitanya Kulkarni <chaitanya.kulkarni@wdc.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20210602100659.11058-1-colin.king@canonical.comSigned-off-by: Jens Axboe <axboe@kernel.dk>

drivers/block/null_blk/main.c

@@ -1856,7 +1856,7 @@ static int null_add_dev(struct nullb_device *dev)
 			goto out_cleanup_tags;
 		nullb->disk = alloc_disk_node(1, nullb->dev->home_node);
 		if (!nullb->disk)
-			goto out_cleanup_disk;
+			goto out_cleanup_tags;
 		nullb->disk->queue = nullb->q;
 	} else if (dev->queue_mode == NULL_Q_BIO) {
 		rv = -ENOMEM;