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 <[e-mail]>
Cc: Martin Schwidefsky <[e-mail]>
Cc: John Stultz <[e-mail]>
Cc: Daniel Walker <[e-mail]>
LKML-Reference: <[e-mail]>
Signed-off-by: Ingo Molnar <[e-mail]>

kernel/time/timekeeping.c

···
  */
 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);
 }