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

This reverts commit 84cfb6ab484b442d5115eb3baf9db7d74a3ea626.  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>
diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index 7bf3ce0..678a22c 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -1091,12 +1091,16 @@
 			cgrp->subsys[i]->cgroup = cgrp;
 			list_move(&ss->sibling, &root->subsys_list);
 			ss->root = root;
+			if (ss->bind)
+				ss->bind(cgrp);
 			/* refcount was already taken, and we're keeping it */
 		} else if (bit & removed_mask) {
 			/* We're removing this subsystem */
 			BUG_ON(ss == NULL);
 			BUG_ON(cgrp->subsys[i] != dummytop->subsys[i]);
 			BUG_ON(cgrp->subsys[i]->cgroup != cgrp);
+			if (ss->bind)
+				ss->bind(dummytop);
 			dummytop->subsys[i]->cgroup = dummytop;
 			cgrp->subsys[i] = NULL;
 			subsys[i]->root = &rootnode;