cpuset: allow to keep tasks in empty cpusets

To achieve this:

- We call update_tasks_cpumask/nodemask() for empty cpusets when
hotplug happens, instead of moving tasks out of them.

- When a cpuset's masks are changed by writing cpuset.cpus/mems,
we also update tasks in child cpusets which are empty.

v3:
- do propagation work in one place for both hotplug and unplug

v2:
- drop rcu_read_lock before calling update_task_nodemask() and
  update_task_cpumask(), instead of using workqueue.
- add documentation in include/linux/cgroup.h

Signed-off-by: Li Zefan <lizefan@huawei.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

include/linux/cgroup.h

@@ -277,6 +277,10 @@ enum {
 	 *
 	 * - Remount is disallowed.
 	 *
+	 * - cpuset: tasks will be kept in empty cpusets when hotplug happens
+	 *   and take masks of ancestors with non-empty cpus/mems, instead of
+	 *   being moved to an ancestor.
+	 *
 	 * - memcg: use_hierarchy is on by default and the cgroup file for
 	 *   the flag is not created.
 	 *