cpu/hotplug: Prevent self deadlock on CPU hot-unplug

Xiongfeng reported and debugged a self deadlock of the task which initiates
and controls a CPU hot-unplug operation vs. the CFS bandwidth timer.

    CPU1      			                 	 CPU2

T1 sets cfs_quota
   starts hrtimer cfs_bandwidth 'period_timer'
T1 is migrated to CPU2				
						T1 initiates offlining of CPU1
Hotplug operation starts
  ...
'period_timer' expires and is re-enqueued on CPU1
  ...
take_cpu_down()
  CPU1 shuts down and does not handle timers
  anymore. They have to be migrated in the
  post dead hotplug steps by the control task.

						T1 runs the post dead offline operation
					      	T1 is scheduled out
						T1 waits for 'period_timer' to expire

T1 waits there forever if it is scheduled out before it can execute the hrtimer
offline callback hrtimers_dead_cpu().

Cure this by delegating the hotplug control operation to a worker thread on
an online CPU. This takes the initiating user space task, which might be
affected by the bandwidth timer, completely out of the picture.

Reported-by: Xiongfeng Wang <wangxiongfeng2@huawei.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Tested-by: Yu Liao <liaoyu15@huawei.com>
Acked-by: Vincent Guittot <vincent.guittot@linaro.org>
Cc: stable@vger.kernel.org
Link: https://lore.kernel.org/lkml/8e785777-03aa-99e1-d20e-e956f5685be6@huawei.com
Link: https://lore.kernel.org/r/87h6oqdq0i.ffs@tglx