KVM: s390: Limit guest size to 16TB

Currently we fill up a full 5 level page table to hold the guest
mapping. Since commit "support gmap page tables with less than 5
levels" we can do better.
Having more than 4 TB might be useful for some testing scenarios,
so let's just limit ourselves to 16TB guest size.
Having more than that is totally untested as I do not have enough
swap space/memory.

We continue to allow ucontrol the full size.
Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
Acked-by: Cornelia Huck <cornelia.huck@de.ibm.com>
Cc: Martin Schwidefsky <schwidefsky@de.ibm.com>

arch/s390/kvm/kvm-s390.c

@@ -458,7 +458,7 @@ int kvm_arch_init_vm(struct kvm *kvm, unsigned long type)
 	if (type & KVM_VM_S390_UCONTROL) {
 		kvm->arch.gmap = NULL;
 	} else {
-		kvm->arch.gmap = gmap_alloc(current->mm, -1UL);
+		kvm->arch.gmap = gmap_alloc(current->mm, (1UL << 44) - 1);
 		if (!kvm->arch.gmap)
 			goto out_nogmap;
 		kvm->arch.gmap->private = kvm;