Each namespace has an owning user namespace and now there is not way
to discover these relationships.

Understending namespaces relationships allows to answer the question:
what capability does process X have to perform operations on a resource
governed by namespace Y?

After a long discussion, Eric W. Biederman proposed to use ioctl-s for
this purpose.

The NS_GET_USERNS ioctl returns a file descriptor to an owning user
namespace.
It returns EPERM if a target namespace is outside of a current user
namespace.

v2: rename parent to relative

v3: Add a missing mntput when returning -EAGAIN --EWB
Acked-by: NSerge Hallyn <serge@hallyn.com>
Link: https://lkml.org/lkml/2016/7/6/158Signed-off-by: NAndrei Vagin <avagin@openvz.org>
Signed-off-by: NEric W. Biederman <ebiederm@xmission.com>