Fix renaming into root directory

The VFS performs a lot of checks on renames before calling the fs
method.  We acquire locks and refresh inodes in the rename method so we
have to duplciate a lot of the vfs checks.

One of the checks involves loops with ancestors and subdirectories.  We
missed the case where the root directory is the destination and doesn't
have any parent directories.  The backref walker it calls returns
-ENOENT instead of 0 with an empty set of parents and that error bubbled
up to rename.

The fix is to notice when we're asking for ancestors of the one
directory that can't have ancestors and short circuit the test.

Signed-off-by: Zach Brown <zab@versity.com>

kmod/src/dir.c

@@ -1443,6 +1443,11 @@ static int item_d_ancestor(struct super_block *sb, u64 p1, u64 p2, u64 *p_ret)
 
 	*p_ret = 0;
 
+	if (p2 == SCOUTFS_ROOT_INO) {
+		ret = 0;
+		goto out;
+	}
+
 	ret = scoutfs_dir_get_backref_path(sb, p2, 0, 0, &list);
 	if (ret)
 		goto out;