cls_cgroup: Fix build error when built-in

There is a typo in cgroup_cls_state when cls_cgroup is built-in.

Signed-off-by: Herbert Xu <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

include/net/cls_cgroup.h

···
 		return 0;
 
 	return container_of(task_subsys_state(p, net_cls_subsys_id),
-			    struct cgroup_cls_state, css).classid;
+			    struct cgroup_cls_state, css)->classid;
 }
 #else
 extern int net_cls_subsys_id;