seize: don't detach from a task if a freezer cgroup is set

criu detaches from tasks in freezer_detach() in this case

Signed-off-by: Andrew Vagin <avagin@virtuozzo.com>
Acked-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>