KVM: mmu: Separate updating a PTE from kvm_set_pte_rmapp

The TDP MMU's own function for the changed-PTE notifier will need to be
update a PTE in the exact same way as the shadow MMU.  Rather than
re-implementing this logic, factor the SPTE creation out of kvm_set_pte_rmapp.

Extracted out of a patch by Ben Gardon. <bgardon@google.com>

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>