Reorganize hotplug path to prepare for async hotplug handling.

* Both CPU and memory hotplug handlings are collected into a single
  function - cpuset_handle_hotplug().  It doesn't take any argument
  but compares the current setttings of top_cpuset against what's
  actually available to determine what happened.  This function
  directly updates top_cpuset.  If there are CPUs or memory nodes
  which are taken down, cpuset_propagate_hotplug() in invoked on all
  !root cpusets.

* cpuset_propagate_hotplug() is responsible for updating the specified
  cpuset so that it doesn't include any resource which isn't available
  to top_cpuset.  If no CPU or memory is left after update, all tasks
  are moved to the nearest ancestor with both resources.

* update_tasks_cpumask() and update_tasks_nodemask() are now always
  called after cpus or mems masks are updated even if the cpuset
  doesn't have any task.  This is for brevity and not expected to have
  any measureable effect.

* cpu_active_mask and N_HIGH_MEMORY are read exactly once per
  cpuset_handle_hotplug() invocation, all cpusets share the same view
  of what resources are available, and cpuset_handle_hotplug() can
  handle multiple resources going up and down.  These properties will
  allow async operation.

The reorganization, while drastic, is equivalent and shouldn't cause
any behavior difference.  This will enable making hotplug handling
async and remove get_online_cpus() -> cgroup_mutex nesting.
Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Li Zefan <lizefan@huawei.com>