cgroup: fix a typo in comment

In commit 8699b77 ("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>

include/linux/cgroup-defs.h

@@ -413,7 +413,7 @@ struct cgroup {
 	/*
 	 * The bitmask of subsystems enabled on the child cgroups.
 	 * ->subtree_control is the one configured through
-	 * "cgroup.subtree_control" while ->child_ss_mask is the effective
+	 * "cgroup.subtree_control" while ->subtree_ss_mask is the effective
 	 * one which may have more subsystems enabled.  Controller knobs
 	 * are made available iff it's enabled in ->subtree_control.
 	 */