s390/time,vdso: fix clock_gettime for CLOCK_MONOTONIC

With git commit 79c74ecb
"s390/time,vdso: convert to the new update_vsyscall interface"
the new update_vsyscall function already does the sum of xtime
and wall_to_monotonic. The old update_vsyscall function only
copied the wall_to_monotonic offset. The vdso code needs to be
modified to take this into consideration.

Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>