KVM: s390: protvirt: Secure memory is not mergeable

KVM: s390: protvirt: Secure memory is not mergeable

KSM will not work on secure pages, because when the kernel reads a
secure page, it will be encrypted and hence no two pages will look the
same.

Let's mark the guest pages as unmergeable when we transition to secure
mode.

Signed-off-by: Janosch Frank <frankja@linux.ibm.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Cornelia Huck <cohuck@redhat.com>
[borntraeger@de.ibm.com: patch merging, splitting, fixing]
Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>