cgroup, legacy_freezer: update comment for freezer_css_offline()

After commit f5d39b02

 ("freezer,sched: Rewrite core freezer logic"),
system_freezing_count was replaced by freezer_active.

Signed-off-by: Xiu Jianfeng <xiujianfeng@huawei.com>
Signed-off-by: Tejun Heo <tj@kernel.org>