workqueue: fix is_chained_work() regression

c9e7cf273f ("workqueue: move busy_hash from global_cwq to
worker_pool") incorrectly converted is_chained_work() to use
get_gcwq() inside for_each_gcwq_cpu() while removing get_gcwq().

As cwq might not exist for all possible workqueue CPUs, @cwq can be
NULL and the following cwq deferences can lead to oops.

Fix it by using for_each_cwq_cpu() instead, which is the better one to
use anyway as we only need to check pools that the wq is associated
with.

Signed-off-by: Tejun Heo <tj@kernel.org>

kernel/workqueue.c

@@ -1167,7 +1167,7 @@ static bool is_chained_work(struct workqueue_struct *wq)
 	unsigned long flags;
 	unsigned int cpu;
 
-	for_each_wq_cpu(cpu) {
+	for_each_cwq_cpu(cpu, wq) {
 		struct cpu_workqueue_struct *cwq = get_cwq(cpu, wq);
 		struct worker_pool *pool = cwq->pool;
 		struct worker *worker;