alarmtimer: Fix unavailable wake-up source in sysfs

Currently the alarmtimer registers a wake-up source unconditionally,
regardless of the system having a (wake-up capable) RTC or not.
Hence the alarmtimer will always show up in
/sys/kernel/debug/wakeup_sources, even if it is not available, and thus
cannot be a wake-up source.

To fix this, postpone registration until a wake-up capable RTC device is
added.

Cc: Thomas Gleixner <[e-mail]>
Cc: Ingo Molnar <[e-mail]>
Cc: Miroslav Lichvar <[e-mail]>
Cc: Richard Cochran <[e-mail]>
Cc: Prarit Bhargava <[e-mail]>
Cc: Stephen Boyd <[e-mail]>
Signed-off-by: Geert Uytterhoeven <[e-mail]>
Signed-off-by: John Stultz <[e-mail]>

kernel/time/alarmtimer.c

···
 static DEFINE_SPINLOCK(freezer_delta_lock);
 #endif
 
+#ifdef CONFIG_RTC_CLASS
 static struct wakeup_source *ws;
 
-#ifdef CONFIG_RTC_CLASS
 /* rtc timer and device for setting alarm wakeups at suspend */
 static struct rtc_timer		rtctimer;
 static struct rtc_device	*rtcdev;
···
 {
 	unsigned long flags;
 	struct rtc_device *rtc = to_rtc_device(dev);
+	struct wakeup_source *__ws;
 
 	if (rtcdev)
 		return -EBUSY;
···
 	if (!device_may_wakeup(rtc->dev.parent))
 		return -1;
 
+	__ws = wakeup_source_register("alarmtimer");
+
 	spin_lock_irqsave(&rtcdev_lock, flags);
 	if (!rtcdev) {
 		rtcdev = rtc;
 		/* hold a reference so it doesn't go away */
 		get_device(dev);
+		ws = __ws;
+		__ws = NULL;
 	}
 	spin_unlock_irqrestore(&rtcdev_lock, flags);
+
+	wakeup_source_unregister(__ws);
+
 	return 0;
 }
 
···
 		error = PTR_ERR(pdev);
 		goto out_drv;
 	}
-	ws = wakeup_source_register("alarmtimer");
 	return 0;
 
 out_drv: