[S390] fix idle time accounting

The steal time is calculated by subtracting the time the virtual cpu
has been running on a physical cpu from the wall clock time. To make
that work all wall time needs to be added to the steal time field first
before the virtual cpu time is subtracted.

The time between the last clock update and the load of the enabled wait
psw needs to be added to the steal_time field as well to make the sum
over all cpu accounting numbers match the wall clock.

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