unseize_task_and_threads sends SIGKILL in unseize_task under condition
(st == TASK_DEAD). Which obviously kills task.

Move freezer_detach after pstree_wait to detach only from alive tasks
to get rid of the following errors:
	(00.242163) Error (seize.c:223): Unable to detach from 23064
	: No such process
	(00.242177) Error (seize.c:223): Unable to detach from 23065
	: No such process
Signed-off-by: Dmitry Safonov <dsafonov@odin.com>
Acked-by: Andrew Vagin <avagin@virtuozzo.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>