s390/pai: Calculate size of reserved PAI extension control block area

s390/pai: Calculate size of reserved PAI extension control block area

The PAI extension 1 control block area is 512 bytes in total.
It currently contains three address pointer which refer to counter
memory blocks followed by a reserved area.
Calculate the reserved area instead of hardcoding its size. This
makes the code more readable and maintainable.
No functional chance.

Signed-off-by: Thomas Richter <tmricht@linux.ibm.com>
Suggested-by: Jan Polensky <japo@linux.ibm.com>
Reviewed-by: Jan Polensky <japo@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>