workqueue: use hotcpu_notifier() for workqueue_cpu_down_callback()

workqueue_cpu_down_callback() is used only if HOTPLUG_CPU=y, so
hotcpu_notifier() fits better than cpu_notifier().

When HOTPLUG_CPU=y, hotcpu_notifier() and cpu_notifier() are the same.

When HOTPLUG_CPU=n, if we use cpu_notifier(),
workqueue_cpu_down_callback() will be called during boot to do
nothing, and the memory of workqueue_cpu_down_callback() and
gcwq_unbind_fn() will be discarded after boot.

If we use hotcpu_notifier(), we can avoid the no-op call of
workqueue_cpu_down_callback() and the memory of
workqueue_cpu_down_callback() and gcwq_unbind_fn() will be discard at
build time:

$ ls -l kernel/workqueue.o.cpu_notifier kernel/workqueue.o.hotcpu_notifier
-rw-rw-r-- 1 laijs laijs 484080 Sep 15 11:31 kernel/workqueue.o.cpu_notifier
-rw-rw-r-- 1 laijs laijs 478240 Sep 15 11:31 kernel/workqueue.o.hotcpu_notifier

$ size kernel/workqueue.o.cpu_notifier kernel/workqueue.o.hotcpu_notifier
   text	   data	    bss	    dec	    hex	filename
  18513	   2387	   1221	  22121	   5669	kernel/workqueue.o.cpu_notifier
  18082	   2355	   1221	  21658	   549a	kernel/workqueue.o.hotcpu_notifier

tj: Updated description.
Signed-off-by: Lai Jiangshan <laijs@cn.fujitsu.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

kernel/workqueue.c

@@ -3807,7 +3807,7 @@ static int __init init_workqueues(void)
 		     WORK_CPU_LAST);
 
 	cpu_notifier(workqueue_cpu_up_callback, CPU_PRI_WORKQUEUE_UP);
-	cpu_notifier(workqueue_cpu_down_callback, CPU_PRI_WORKQUEUE_DOWN);
+	hotcpu_notifier(workqueue_cpu_down_callback, CPU_PRI_WORKQUEUE_DOWN);
 
 	/* initialize gcwqs */
 	for_each_gcwq_cpu(cpu) {