cgroup: Do not report unavailable v1 controllers in /proc/cgroups

This is a followup to CONFIG-urability of cpuset and memory controllers
for v1 hierarchies. Make the output in /proc/cgroups reflect that
!CONFIG_CPUSETS_V1 is like !CONFIG_CPUSETS and
!CONFIG_MEMCG_V1 is like !CONFIG_MEMCG.

The intended effect is that hiding the unavailable controllers will hint
users not to try mounting them on v1.
Signed-off-by: Michal Koutný <mkoutny@suse.com>
Reviewed-by: Waiman Long <longman@redhat.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

kernel/cgroup/cgroup-v1.c

@@ -681,11 +681,14 @@ int proc_cgroupstats_show(struct seq_file *m, void *v)
 	 * cgroup_mutex contention.
 	 */
 
-	for_each_subsys(ss, i)
+	for_each_subsys(ss, i) {
+		if (cgroup1_subsys_absent(ss))
+			continue;
 		seq_printf(m, "%s\t%d\t%d\t%d\n",
 			   ss->legacy_name, ss->root->hierarchy_id,
 			   atomic_read(&ss->root->nr_cgrps),
 			   cgroup_ssid_enabled(i));
+	}
 
 	return 0;
 }