Return full butter path from butter root.

def fullPath(self):
        """ Return full butter path from butter root. """
        for ((dirTree, dirID, dirSeq), (dirPath, name)) in self.links.items():
            try:
                path = self.fileSystem.volumes[dirTree].fullPath
                if path is not None:
                    return path + ("/" if path[-1] != "/" else "") + dirPath + name
            except Exception:
                logging.debug("Haven't imported %d yet", dirTree)

        if self.id == BTRFS_FS_TREE_OBJECTID:
            return "/"
        else:
            return None