cgroups: save freezer state during dump

CRIU sets freezer.state to "THAWED" during process tree dumping. That's why
we can't simply save freezer.state file contents to cgroups image. New
special function get_real_freezer_state() returns freezer cgroup state
observed before CRIU dumping start. Patch puts its return value to dump file.

Signed-off-by: Evgeniy Akimov <geka666@gmail.com>
Signed-off-by: Eugene Batalov <eabatalov89@gmail.com>
Acked-by: Andrew Vagin <avagin@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>