If multiple cgroups are mounted under /sys/fs/cgroup, then the
original check ends up looking for /sys/fs/cgroup/cgroup.clone_children,
which does not exist because that is just a tmpfs.

So make sure to check an actual cgroupfs.
Signed-off-by: Serge Hallyn <serge.hallyn@canonical.com>
Signed-off-by: Daniel Lezcano <dlezcano@fr.ibm.com>