cgroup/cpuset: move legacy hotplug update to cpuset-v1.c

There are some differents about hotplug update between cpuset v1 and
cpuset v2. Move the legacy code to cpuset-v1.c.

'update_tasks_cpumask' and 'update_tasks_nodemask' are both used in cpuset
v1 and cpuset v2, declare them in cpuset-internal.h.

The change from original code is that use callback_lock helpers to get
callback_lock lock/unlock.

Signed-off-by: Chen Ridong <chenridong@huawei.com>
Acked-by: Waiman Long <longman@redhat.com>
Signed-off-by: Tejun Heo <tj@kernel.org>