core: tiny tweak for cgroup trimming during manager_free()

core: tiny tweak for cgroup trimming during manager_free()

Instead of blacklisting when not to trim the cgroup tree, let's instead
whitelist when to do it, as an excercise of being careful when being
destructive.

This should not change behaviour with exception that during switch roots
we now won't attempt to trim the cgroup tree anymore. Which is more
correct behaviour after all we serialize/deserialize during the
transition and should be needlessly destructive.