kvm: fix compile on s390 part 2

We also need to fence the memunmap part.

Fixes: e45adf66 ("KVM: Introduce a new guest mapping API")
Fixes: d30b214d (kvm: fix compilation on s390)
Cc: Michal Kubecek <mkubecek@suse.cz>
Cc: KarimAllah Ahmed <karahmed@amazon.de>
Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>