CVE-2023-52639 KVM: s390: vsie: fix race during shadow creation

In the Linux kernel, the following vulnerability has been resolved:

KVM: s390: vsie: fix race during shadow creation

Right now it is possible to see gmap->private being zero in
kvm_s390_vsie_gmap_notifier resulting in a crash. This is due to the
fact that we add gmap->private == kvm after creation:

static int acquire_gmap_shadow(struct kvm_vcpu *vcpu,
struct vsie_page *vsie_page)
{
[…]
gmap = gmap_shadow(vcpu->arch.gmap, asce, edat);
if (IS_ERR(gmap))
return PTR_ERR(gmap);
gmap->private = vcpu->kvm;

Let children inherit the private field of the parent.