cgroup/freezer: Reduce redundant traversal for cgroup_freeze

Whether a cgroup is frozen is determined solely by whether it is set to
to be frozen and whether its parent is frozen. Currently, when is cgroup
is frozen or unfrozen, it iterates through the entire subtree to freeze
or unfreeze its descentdants. However, this is unesessary for a cgroup
that does not change its effective frozen status. This path aims to skip
the subtree if its parent does not have a change in effective freeze.

For an example, subtree like, a-b-c-d-e-f-g, when a is frozen, the
entire tree is frozen. If we freeze b and c again, it is unesessary to
iterate d, e, f and g. So does that If we unfreeze b/c.

Reviewed-by: Michal Koutný <mkoutny@suse.com>
Signed-off-by: Chen Ridong <chenridong@huawei.com>
Signed-off-by: Tejun Heo <tj@kernel.org>