KVM: s390: ignore IBC if zero

Looks like we forgot about the special IBC value of 0 meaning "no IBC".
Let's fix that, otherwise it gets rounded up and suddenly an IBC is active
with the lowest possible machine.
Signed-off-by: David Hildenbrand <dahi@linux.vnet.ibm.com>
Reviewed-by: Cornelia Huck <cornelia.huck@de.ibm.com>
Fixes: commit 053dd230 ("KVM: s390: force ibc into valid range")
Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>

arch/s390/kvm/kvm-s390.c

@@ -657,7 +657,7 @@ static int kvm_s390_set_processor(struct kvm *kvm, struct kvm_device_attr *attr)
 		kvm->arch.model.cpuid = proc->cpuid;
 		lowest_ibc = sclp.ibc >> 16 & 0xfff;
 		unblocked_ibc = sclp.ibc & 0xfff;
-		if (lowest_ibc) {
+		if (lowest_ibc && proc->ibc) {
 			if (proc->ibc > unblocked_ibc)
 				kvm->arch.model.ibc = unblocked_ibc;
 			else if (proc->ibc < lowest_ibc)