timekeeping: fix 32-bit overflow in get_monotonic_boottime

fixed upstream in v3.6 by ec145bab

get_monotonic_boottime adds three nanonsecond values stored
in longs, followed by an s64.  If the long values are all
close to 1e9 the first three additions can overflow and
become negative when added to the s64.  Cast the first
value to s64 so that all additions are 64 bit.
Signed-off-by: Colin Cross <ccross@android.com>
[jstultz: Fished this out of the AOSP commong.git tree. This was
fixed upstream in v3.6 by ec145bab]
Signed-off-by: John Stultz <john.stultz@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

kernel/time/timekeeping.c

@@ -1161,7 +1161,7 @@ void get_monotonic_boottime(struct timespec *ts)
 	} while (read_seqretry(&xtime_lock, seq));
 
 	set_normalized_timespec(ts, ts->tv_sec + tomono.tv_sec + sleep.tv_sec,
-			ts->tv_nsec + tomono.tv_nsec + sleep.tv_nsec + nsecs);
+		(s64)ts->tv_nsec + tomono.tv_nsec + sleep.tv_nsec + nsecs);
 }
 EXPORT_SYMBOL_GPL(get_monotonic_boottime);