[0/3] cgroup namespace update

[Serge Hallyn]

Hi.  During xenial cycle we applied a patch (and a followon fix for criu)
to fix a usability problem with cgroup namespaces and /proc/self/mountinfo.
Since then, the approach to fix it upstream has changed.  Following are
reverts for the two patches, plus the new patch cherrypicked from the
official cgroup for-4.6 tree.

Impact:
One or the other of these patchsets suffices for docker in lxd to work.
The patchset being reverted required a docker patch, the new one does
not.  Docker cannot work inside lxd with neither patchset.


thanks,
-serge