cgroup: fix a typo in comment

In commit 8699b776 ("cgroup: s/child_subsys_mask/subtree_ss_mask/"),
we rename child_subsys_mask to subtree_ss_mask. While it missed to
rename this in comment.

Signed-off-by: Wei Yang <richard.weiyang@gmail.com>
Signed-off-by: Tejun Heo <tj@kernel.org>