Btrfs: Add locking around volume management (device add/remove/balance)

Signed-off-by: Chris Mason <chris.mason@oracle.com>

fs/btrfs/ctree.h

@@ -523,6 +523,7 @@ struct btrfs_fs_info {
 	struct mutex alloc_mutex;
 	struct mutex chunk_mutex;
 	struct mutex drop_mutex;
+	struct mutex volume_mutex;
 	struct list_head trans_list;
 	struct list_head hashers;
 	struct list_head dead_roots;