Pid and user namepaces are hierarchical. There is no way to discover
parent-child relationships.

In a future we will use this interface to dump and restore nested
namespaces.
Acked-by: NSerge Hallyn <serge@hallyn.com>
Signed-off-by: NAndrei Vagin <avagin@openvz.org>
Signed-off-by: NEric W. Biederman <ebiederm@xmission.com>