cgroup: fix offlining child waiting in cgroup_subtree_control_write()

cgroup_subtree_control_write() waits for offline to complete
child-by-child before enabling a controller; however, it has a couple
bugs.

* It doesn't initialize the wait_queue_t.  This can lead to infinite
  hang on the following schedule() among other things.

* It forgets to pin the child before releasing cgroup_tree_mutex and
  performing schedule().  The child may already be gone by the time it
  wakes up and invokes finish_wait().  Pin the child being waited on.
Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Li Zefan <lizefan@huawei.com>

kernel/cgroup.c

@@ -2594,16 +2594,18 @@ static int cgroup_subtree_control_write(struct cgroup_subsys_state *dummy_css,
 			 * cases, wait till it's gone using offline_waitq.
 			 */
 			cgroup_for_each_live_child(child, cgrp) {
-				wait_queue_t wait;
+				DEFINE_WAIT(wait);
 
 				if (!cgroup_css(child, ss))
 					continue;
 
+				cgroup_get(child);
 				prepare_to_wait(&child->offline_waitq, &wait,
 						TASK_UNINTERRUPTIBLE);
 				mutex_unlock(&cgroup_tree_mutex);
 				schedule();
 				finish_wait(&child->offline_waitq, &wait);
+				cgroup_put(child);
 				goto retry;
 			}