--freeze-cgroup: should also seize tasks in sub-cgroups

Signed-off-by: Tycho Andersen <tycho.andersen@canonical.com>
CC: Andrew Vagin <avagin@virtuozzo.com>
Acked-by: Andrew Vagin <avagin@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>