workqueue: fix incorrect cpu number BUG_ON() in get_work_gcwq()

get_work_gcwq() was incorrectly triggering BUG_ON() if cpu number is
equal to or higher than num_possible_cpus() instead of nr_cpu_ids.
Fix it.
Signed-off-by: Tejun Heo <tj@kernel.org>

kernel/workqueue.c

@@ -445,7 +445,7 @@ static struct global_cwq *get_work_gcwq(struct work_struct *work)
 	if (cpu == NR_CPUS)
 		return NULL;
 
-	BUG_ON(cpu >= num_possible_cpus());
+	BUG_ON(cpu >= nr_cpu_ids);
 	return get_gcwq(cpu);
 }