Fix del_timer_sync() SMP memory ordering (from Tejun Huh <tejun@aratech.co.kr>)

From Tejun's posting:
>
> This patch fixes a race between del_timer_sync and recursive timers.
> Current implementation allows the value of timer->base that is used
> for timer_pending test to be fetched before finishing running_timer
> test, so it's possible for a recursive time to be pending after
> del_timer_sync.  Adding smp_rmb before timer_pending removes the race.

kernel/timer.c

@@ -338,6 +338,7 @@ int del_timer_sync(struct timer_list *timer)
 			break;
 		}
 	}
+	smp_rmb();
 	if (timer_pending(timer))
 		goto del_again;