cgroup: Split legacy 'ns' cgroup handling off from main cgroup handling

cgroup: Split legacy 'ns' cgroup handling off from main cgroup handling

This patch splits off ns legacy cgroup handling from main cgroup
handling. It moves the creation of the cgroups before clone(), so that
the child will easily know which cgroups it will later belong to. Since
this is not possible for the renaming of the 'ns' cgroup, keep that
part after clone.

Signed-off-by: Christian Seiler <christian@iwakd.de>
Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>