KVM: s390: vsie: fix race during shadow creation

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.

Reported-by: Marc Hartmayer <[email protected]>
Fixes: a3508fbe9dc6 ("KVM: s390: vsie: initial support for nested virtualization")
Cc: <[email protected]>
Cc: David Hildenbrand <[email protected]>
Reviewed-by: Janosch Frank <[email protected]>
Reviewed-by: David Hildenbrand <[email protected]>
Reviewed-by: Claudio Imbrenda <[email protected]>
Signed-off-by: Christian Borntraeger <[email protected]>
Link: https://lore.kernel.org/r/[email protected]