workqueue: Restrict kworker in the offline CPU pool running on housekeeping CPUs

When a CPU is going offline, all workers on the CPU's pool will have their
cpus_allowed cleared to cpu_possible_mask and can run on any CPUs including
the isolated ones. Instead, set cpus_allowed to wq_unbound_cpumask so that
the can avoid isolated CPUs.
Signed-off-by: Zqiang <qiang1.zhang@intel.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

kernel/workqueue.c

@@ -5001,7 +5001,7 @@ static void unbind_workers(int cpu)
 
 		for_each_pool_worker(worker, pool) {
 			kthread_set_per_cpu(worker->task, -1);
-			WARN_ON_ONCE(set_cpus_allowed_ptr(worker->task, cpu_possible_mask) < 0);
+			WARN_ON_ONCE(set_cpus_allowed_ptr(worker->task, wq_unbound_cpumask) < 0);
 		}
 
 		mutex_unlock(&wq_pool_attach_mutex);