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

Analog to git commit 28b92e09
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>