alarmtimer: Implement try_to_cancel callback

Preparatory change to utilize the common posix timer mechanisms.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: John Stultz <john.stultz@linaro.org>
Link: http://lkml.kernel.org/r/20170530211657.670026824@linutronix.de

kernel/time/alarmtimer.c

@@ -572,6 +572,15 @@ static ktime_t alarm_timer_remaining(struct k_itimer *timr, ktime_t now)
 	return ktime_sub(now, alarm->node.expires);
 }
 
+/**
+ * alarm_timer_try_to_cancel - Posix timer callback to cancel a timer
+ * @timr:	Pointer to the posixtimer data struct
+ */
+static int alarm_timer_try_to_cancel(struct k_itimer *timr)
+{
+	return alarm_try_to_cancel(&timr->it.alarm.alarmtimer);
+}
+
 /**
  * alarm_clock_getres - posix getres interface
  * @which_clock: clockid
@@ -902,6 +911,7 @@ const struct k_clock alarm_clock = {
 	.timer_rearm		= alarm_timer_rearm,
 	.timer_forward		= alarm_timer_forward,
 	.timer_remaining	= alarm_timer_remaining,
+	.timer_try_to_cancel	= alarm_timer_try_to_cancel,
 	.nsleep			= alarm_timer_nsleep,
 };
 #endif /* CONFIG_POSIX_TIMERS */