dm: fix a crash if blk_alloc_disk fails

If blk_alloc_disk fails, the variable md->disk is set to an error value.
cleanup_mapped_device will see that md->disk is non-NULL and it will
attempt to access it, causing a crash on this statement
"md->disk->private_data = NULL;".

Signed-off-by: Mikulas Patocka <[e-mail]>
Reported-by: Chenyuan Yang <[e-mail]>
Closes: https://marc.info/?l=dm-devel&m=172824125004329&w=2
Cc: [e-mail]
Reviewed-by: Nitesh Shetty <[e-mail]>

drivers/md/dm.c

···
 	 * override accordingly.
 	 */
 	md->disk = blk_alloc_disk(NULL, md->numa_node_id);
-	if (IS_ERR(md->disk))
+	if (IS_ERR(md->disk)) {
+		md->disk = NULL;
 		goto bad;
+	}
 	md->queue = md->disk->queue;
 
 	init_waitqueue_head(&md->wait);