cgroupfs: support older kernels without cgroup.clone_children

When creating a cgroup, detect whether cgroup.clone_children
exists.  If not, then manually copy the parent's cpuset.cpus
and cpuset.mems values.

Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>
Acked-by: Stéphane Graber <stgraber@ubuntu.com>