seize: detach from all processes before waiting the root task

We need to detach from all processes before waiting the root
task, because one of these processes may collect processes from a
target pid namespace. The pid namespace is destroyed only when all
processes have been killed and collected.

https://jira.sw.ru/browse/PSBM-43089


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