itimers: Handle relative timers with CONFIG_TIME_LOW_RES proper

As Helge reported for timerfd we have the same issue in itimers. We return
remaining time larger than the programmed relative time to user space in case
of CONFIG_TIME_LOW_RES=y. Use the proper function to adjust the extra time
added in hrtimer_start_range_ns().
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Helge Deller <deller@gmx.de>
Cc: John Stultz <john.stultz@linaro.org>
Cc: linux-m68k@lists.linux-m68k.org
Cc: dhowells@redhat.com
Cc: stable@vger.kernel.org
Link: http://lkml.kernel.org/r/20160114164159.528222587@linutronix.deSigned-off-by: Thomas Gleixner <tglx@linutronix.de>

kernel/time/itimer.c

@@ -26,7 +26,7 @@
  */
 static struct timeval itimer_get_remtime(struct hrtimer *timer)
 {
-	ktime_t rem = hrtimer_get_remaining(timer);
+	ktime_t rem = __hrtimer_get_remaining(timer, true);
 
 	/*
 	 * Racy but safe: if the itimer expires after the above