cgroup: build list of all cgroups under a given cgroupfs_root

Build a list of all cgroups anchored at cgroupfs_root->allcg_list and
going through cgroup->allcg_node.  The list is protected by
cgroup_mutex and will be used to improve cgroup file handling.

Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Li Zefan <lizf@cn.fujitsu.com>
diff --git a/include/linux/cgroup.h b/include/linux/cgroup.h
index 5a85b34..ad2a146 100644
--- a/include/linux/cgroup.h
+++ b/include/linux/cgroup.h
@@ -191,6 +191,8 @@
 	 */
 	struct list_head css_sets;
 
+	struct list_head allcg_node;	/* cgroupfs_root->allcg_list */
+
 	/*
 	 * Linked list running through all cgroups that can
 	 * potentially be reaped by the release agent. Protected by