kvm/s390: also set guest pages back to stable on kexec/kdump

We need to reset the usage state of the pages on kexec/kdump,
which use subcode 0 and 1. We will only do the cmma reset in
the kernel, everything else is done in userspace as before.

Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>