alarmtimer: don't rate limit one-shot timers

BugLink: http://bugs.launchpad.net/bugs/1707233

Commit ff86bf0c ("alarmtimer: Rate limit periodic intervals") sets a
minimum bound on the alarm timer interval.  This minimum bound shouldn't
be applied if the interval is 0.  Otherwise, one-shot timers will be
converted into periodic ones.

Fixes: ff86bf0c ("alarmtimer: Rate limit periodic intervals")
Reported-by: Ben Fennema <fennema@google.com>
Signed-off-by: Greg Hackmann <ghackmann@google.com>
Cc: stable@vger.kernel.org
Cc: John Stultz <john.stultz@linaro.org>
Reviewed-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Stefan Bader <stefan.bader@canonical.com>
Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo@canonical.com>

kernel/time/alarmtimer.c

@@ -616,7 +616,8 @@ static int alarm_timer_set(struct k_itimer *timr, int flags,
 	 * Rate limit to the tick as a hot fix to prevent DOS. Will be
 	 * mopped up later.
 	 */
-	if (ktime_to_ns(timr->it.alarm.interval) < TICK_NSEC)
+	if (timr->it.alarm.interval.tv64 &&
+			ktime_to_ns(timr->it.alarm.interval) < TICK_NSEC)
 		timr->it.alarm.interval = ktime_set(0, TICK_NSEC);
 
 	exp = timespec_to_ktime(new_setting->it_value);