KVM: s390: vsie: clarifications on setting the APCB

The APCB is part of the CRYCB.
The calculation of the APCB origin can be done by adding
the APCB offset to the CRYCB origin.

Current code makes confusing transformations, converting
the CRYCB origin to a pointer to calculate the APCB origin.

Let's make things simpler and keep the CRYCB origin to make
these calculations.

Signed-off-by: Pierre Morel <pmorel@linux.ibm.com>
Reviewed-by: Claudio Imbrenda <imbrenda@linux.ibm.com>
Acked-by: David Hildenbrand <david@redhat.com>
Acked-by: Janosch Frank <frankja@linux.ibm.com>
Signed-off-by: Janosch Frank <frankja@linux.ibm.com>
Link: https://lore.kernel.org/r/20230214122841.13066-2-pmorel@linux.ibm.com
Message-Id: <20230214122841.13066-2-pmorel@linux.ibm.com>