rtc: handle errors correctly in rtc_irq_set_state()

The code checks the correctness of the parameters, but unconditionally
arms/disarms the hrtimer.

The result is that a random task might arm/disarm rtc timer and surprise
the real owner by either generating events or by stopping them.
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: John Stultz <john.stultz@linaro.org>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Ben Greear <greearb@candelatech.com>
Cc: <stable@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

drivers/rtc/interface.c

@@ -656,6 +656,8 @@ int rtc_irq_set_state(struct rtc_device *rtc, struct rtc_task *task, int enabled
 		err = -EBUSY;
 	if (rtc->irq_task != task)
 		err = -EACCES;
+	if (err)
+		goto out;
 
 	if (enabled) {
 		ktime_t period = ktime_set(0, NSEC_PER_SEC/rtc->irq_freq);
@@ -664,6 +666,7 @@ int rtc_irq_set_state(struct rtc_device *rtc, struct rtc_task *task, int enabled
 		hrtimer_cancel(&rtc->pie_timer);
 	}
 	rtc->pie_enabled = enabled;
+out:
 	spin_unlock_irqrestore(&rtc->irq_task_lock, flags);
 
 	return err;