cgroup: switch to css_is_online() helper

cgroup: switch to css_is_online() helper

Use the new css_is_online() helper that has been introduced to check css
online state, instead of testing the CSS_ONLINE flag directly. This
improves readability and centralizes the state check logic.

No functional changes intended.

Signed-off-by: Chen Ridong <chenridong@huawei.com>
Acked-by: Shakeel Butt <shakeel.butt@linux.dev>
Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Tejun Heo <tj@kernel.org>