Avoid warning when CPU hotplug isn't enabled

Commit e9fb7631 ("cpu-hotplug: introduce cpu_notify(),
__cpu_notify(), cpu_notify_nofail()") also introduced this annoying
warning:

  kernel/cpu.c:157: warning: 'cpu_notify_nofail' defined but not used

when CONFIG_HOTPLUG_CPU wasn't set.

So move that helper inside the #ifdef CONFIG_HOTPLUG_CPU region, and
simplify it while at it.
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

kernel/cpu.c

@@ -154,16 +154,13 @@ static int cpu_notify(unsigned long val, void *v)
 	return __cpu_notify(val, v, -1, NULL);
 }
 
+#ifdef CONFIG_HOTPLUG_CPU
+
 static void cpu_notify_nofail(unsigned long val, void *v)
 {
-	int err;
-
-	err = cpu_notify(val, v);
-	BUG_ON(err);
+	BUG_ON(cpu_notify(val, v));
 }
 
-#ifdef CONFIG_HOTPLUG_CPU
-
 EXPORT_SYMBOL(register_cpu_notifier);
 
 void __ref unregister_cpu_notifier(struct notifier_block *nb)