KVM: s390: randomize sca address

We allocate a page for the 2k sca, so lets use the space to improve
hit rate of some internal cpu caches. No need to change the freeing
of the page, as this will shift away the page offset bits anyway.

Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
Reviewed-by: David Hildenbrand <dahi@linux.vnet.ibm.com>