nbd: add error handling support for add_disk()

We never checked for errors on add_disk() as this function
returned void. Now that this is fixed, use the shiny new
error handling.
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Hannes Reinecke <hare@suse.de>
Signed-off-by: Luis Chamberlain <mcgrof@kernel.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

drivers/block/nbd.c

@@ -1762,7 +1762,9 @@ static struct nbd_device *nbd_dev_add(int index, unsigned int refs)
 	disk->fops = &nbd_fops;
 	disk->private_data = nbd;
 	sprintf(disk->disk_name, "nbd%d", index);
-	add_disk(disk);
+	err = add_disk(disk);
+	if (err)
+		goto out_err_disk;
 
 	/*
 	 * Now publish the device.
@@ -1771,6 +1773,8 @@ static struct nbd_device *nbd_dev_add(int index, unsigned int refs)
 	nbd_total_devices++;
 	return nbd;
 
+out_err_disk:
+	blk_cleanup_disk(disk);
 out_free_idr:
 	mutex_lock(&nbd_index_mutex);
 	idr_remove(&nbd_index_idr, index);