seize: detach from all processes before waiting the root task
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 <74c05afa918858348d6db8037d82fa8124870113@virtuozzo.com>
Acked-by: Cyrill Gorcunov <7a1ea01eee6961eb1e372e3508c2670446d086f4@openvz.org>
Signed-off-by: Pavel Emelyanov <c9a32589e048e044184536f7ac71ef92fe82df3e@virtuozzo.com>

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