timekeeping: Fix invalid getboottime() value

Don't use timespec_add_safe() with wall_to_monotonic, because
wall_to_monotonic has negative values which will cause overflow
in timespec_add_safe(). That makes btime in /proc/stat invalid.
Signed-off-by: Hiroshi Shimamoto <h-shimamoto@ct.jp.nec.com>
Cc: Martin Schwidefsky <schwidefsky@de.ibm.com>
Cc: John Stultz <johnstul@us.ibm.com>
Cc: Daniel Walker <dwalker@fifo99.com>
LKML-Reference: <4A937FDE.4050506@ct.jp.nec.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

kernel/time/timekeeping.c

@@ -826,9 +826,11 @@ void update_wall_time(void)
  */
 void getboottime(struct timespec *ts)
 {
-	struct timespec boottime;
+	struct timespec boottime = {
+		.tv_sec = wall_to_monotonic.tv_sec + total_sleep_time.tv_sec,
+		.tv_nsec = wall_to_monotonic.tv_nsec + total_sleep_time.tv_nsec
+	};
 
-	boottime = timespec_add_safe(wall_to_monotonic, total_sleep_time);
 	set_normalized_timespec(ts, -boottime.tv_sec, -boottime.tv_nsec);
 }