cgroup: convert cgroup_has_live_children() into css_has_online_children()

Now that cgroup liveliness and css onliness are the same state,
convert cgroup_has_live_children() into css_has_online_children() so
that it can be used for actual csses too.  The function now uses
css_for_each_child() for iteration and is published.

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