cgroup: move check_for_release(parent) call to the end of cgroup_destroy_locked()

Currently, check_for_release() on the parent of a destroyed cgroup is
invoked from cgroup_destroy_css_killed().  This is because this is
where the destroyed cgroup can be removed from the parent's children
list.  check_for_release() tests the emptiness of the list directly,
so invoking it before removing the cgroup from the list makes it think
that the parent still has children even when it no longer does.

This patch updates check_for_release() to use
cgroup_has_live_children() instead of directly testing ->children
emptiness and moves check_for_release(parent) earlier to the end of
cgroup_destroy_locked().  As cgroup_has_live_children() ignores
cgroups marked DEAD, check_for_release() functions correctly as long
as it's called after asserting DEAD.

This makes release notification slightly more timely and more
importantly enables further simplification of cgroup destruction path.

Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Li Zefan <lizefan@huawei.com>