cgfsng: handle v1 cpuset hierarchy first

cgfsng: handle v1 cpuset hierarchy first

If the value of cgroup.clone_children in our immediate ancestor cgroup
is 0 then the cpuset of any cgroups we create in subtrees will be empty
and hence we'll copy an empty cpuset at which point we cannot enter the
cpuset cgroup.
Avoid this problem by initializing cgroup.clone_children to 1 an copying
the initialized cpuset of our immediate ancestor. Note, that the cpuset
of our immediate ancestor must be initialized and ours as well otherwise
we couldn't be located in this cgroup.

Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>