workqueue: Put the pwq after detaching the rescuer from the pool

In the Linux kernel, the following vulnerability has been resolved:

workqueue: Put the pwq after detaching the rescuer from the pool

The commit 68f83057b913("workqueue: Reap workers via kthread_stop() and

remove detach_completion") adds code to reap the normal workers but

mistakenly does not handle the rescuer and also removes the code waiting

for the rescuer in put_unbound_pool(), which caused a use-after-free bug

reported by Cheung Wall.

To avoid the use-after-free bug, the pool’s reference must be held until

the detachment is complete. Therefore, move the code that puts the pwq

after detaching the rescuer from the pool.