null_blk: 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. The actual cleanup in case of error is
already handled by the caller of null_gendisk_register().

Signed-off-by: Luis Chamberlain <mcgrof@kernel.org>

drivers/block/null_blk/main.c

@@ -1721,8 +1721,7 @@ static int null_gendisk_register(struct nullb *nullb)
 			return ret;
 	}
 
-	add_disk(disk);
-	return 0;
+	return add_disk(disk);
 }
 
 static int null_init_tag_set(struct nullb *nullb, struct blk_mq_tag_set *set)