Btrfs: Add properly locking around add_root_to_dirty_list

add_root_to_dirty_list happens once at the very beginning of the
transaction, but it is still racey.

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

fs/btrfs/ctree.c

@@ -220,10 +220,12 @@ struct extent_buffer *btrfs_read_lock_root_node(struct btrfs_root *root)
  */
 static void add_root_to_dirty_list(struct btrfs_root *root)
 {
+	spin_lock(&root->fs_info->trans_lock);
 	if (root->track_dirty && list_empty(&root->dirty_list)) {
 		list_add(&root->dirty_list,
 			 &root->fs_info->dirty_cowonly_roots);
 	}
+	spin_unlock(&root->fs_info->trans_lock);
 }
 
 /*