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: Ilya Dryomov <idryomov@gmail.com>
diff --git a/fs/btrfs/extent-tree.c b/fs/btrfs/extent-tree.c
index 4eb7d2b..6cfcc90 100644
--- a/fs/btrfs/extent-tree.c
+++ b/fs/btrfs/extent-tree.c
@@ -7312,7 +7312,7 @@
 		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);