Revert "cgroup: remove bind() method from cgroup_subsys."

This reverts commit 84cfb6ab48.  There
are scheduled changes which make use of the removed callback.

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Rami Rosen <ramirose@gmail.com>
Cc: Li Zefan <lizefan@huawei.com>

include/linux/cgroup.h

@@ -484,6 +484,8 @@ struct cgroup_subsys {
 	void (*fork)(struct task_struct *task);
 	void (*exit)(struct cgroup *cgrp, struct cgroup *old_cgrp,
 		     struct task_struct *task);
+	void (*bind)(struct cgroup *root);
+
 	int subsys_id;
 	int active;
 	int disabled;