s390/topology: move cpumasks away from stack

Make cpumasks static variables to avoid potential large stack
frames. There shouldn't be any concurrent callers since all current
callers are serialized with the cpu hotplug lock.

Reviewed-by: Alexander Gordeev <agordeev@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>