workqueue: workqueue_cpu_callback() should be cpu_notifier instead of hotcpu_notifier

Commit 6ee0578b (workqueue: mark init_workqueues as early_initcall)
made workqueue SMP initialization depend on workqueue_cpu_callback(),
which however was registered as hotcpu_notifier() and didn't get
called if CONFIG_HOTPLUG_CPU is not set.  This made gcwqs on non-boot
CPUs not create their initial workers leading to boot failures.  Fix
it by making it a cpu_notifier.
Signed-off-by: Tejun Heo <tj@kernel.org>
Reported-and-bisected-by: walt <w41ter@gmail.com>
Tested-by: Markus Trippelsdorf <markus@trippelsdorf.de>

kernel/workqueue.c

@@ -3527,7 +3527,7 @@ static int __init init_workqueues(void)
 	unsigned int cpu;
 	int i;
 
-	hotcpu_notifier(workqueue_cpu_callback, CPU_PRI_WORKQUEUE);
+	cpu_notifier(workqueue_cpu_callback, CPU_PRI_WORKQUEUE);
 
 	/* initialize gcwqs */
 	for_each_gcwq_cpu(cpu) {