KVM: s390: Fix KVM_S390_VCPU_FAULT ioctl

KVM: s390: Fix KVM_S390_VCPU_FAULT ioctl

A previous commit changed the behaviour of the KVM_S390_VCPU_FAULT
ioctl. The current (wrong) implementation will trigger a guest
addressing exception if the requested address lies outside of a
memslot, unless the VM is UCONTROL.

Restore the previous behaviour by open coding the fault-in logic.

Fixes: 3762e905ec2e ("KVM: s390: use __kvm_faultin_pfn()")
Acked-by: Christian Borntraeger <borntraeger@linux.ibm.com>
Reviewed-by: Steffen Eiden <seiden@linux.ibm.com>
Signed-off-by: Claudio Imbrenda <imbrenda@linux.ibm.com>