Btrfs: pass the correct root to lookup_free_space_inode()

Free space items are located in tree of tree roots, not in the extent
tree.  It didn't pop up because lookup_free_space_inode() grabs the
inode all the time instead of actually searching the tree.
Signed-off-by: NIlya Dryomov <idryomov@gmail.com>

fs/btrfs/extent-tree.c

@@ -7312,7 +7312,7 @@ int btrfs_remove_block_group(struct btrfs_trans_handle *trans,
 		goto out;
 	}
 
-	inode = lookup_free_space_inode(root, block_group, path);
+	inode = lookup_free_space_inode(tree_root, block_group, path);
 	if (!IS_ERR(inode)) {
 		ret = btrfs_orphan_add(trans, inode);
 		BUG_ON(ret);