[S390] reset cputime accounting after IPL from NSS

After an IPL from NSS the uptime of the system is incorrect. The reason
is that the startup code in head.S is not executed in case of an IPL
from NSS. Due to that sched_clock_base_cc which is used to initialze
wall_to_monotonic contains the time stamp when the NSS has been created
instead of the time stamp of the system start.

Reinitialize the cputime accounting values in create_kernel_nss after
the SAVESYS CP command that created the NSS segment.

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