code: private void unprotectedMkdir(long inodeId, INode[] inodes, int pos,
        byte[] name, PermissionStatus permission, boolean inheritPermission,
        long timestamp) throws QuotaExceededException {
      inodes[pos] = addChild(inodes, pos,
          new INodeDirectory(inodeId, name, permission, timestamp),
          -1, inheritPermission );
    }
docs: create a directory at index pos.
  The parent path to the directory is at [0, pos-1].
  All ancestors exist. Newly created one stored at index pos.