s390/time: cast tv_nsec to u64 prior to shift in update_vsyscall

Analog to git commit 28b92e09e25bdc0ae864b22eacf195a74f861389
first cast tk->wall_to_monotonic.tv_nsec to u64 before doing
the shift with tk->shift to avoid loosing relevant bits on a
32-bit kernel.

Cc: stable@vger.kernel.org # 3.13+
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>

arch/s390/kernel/time.c

@@ -226,7 +226,7 @@ void update_vsyscall(struct timekeeper *tk)
 	vdso_data->wtom_clock_sec =
 		tk->xtime_sec + tk->wall_to_monotonic.tv_sec;
 	vdso_data->wtom_clock_nsec = tk->xtime_nsec +
-		+ (tk->wall_to_monotonic.tv_nsec << tk->shift);
+		+ ((u64) tk->wall_to_monotonic.tv_nsec << tk->shift);
 	nsecps = (u64) NSEC_PER_SEC << tk->shift;
 	while (vdso_data->wtom_clock_nsec >= nsecps) {
 		vdso_data->wtom_clock_nsec -= nsecps;