KVM: guest_memfd: do not go through struct page

We have a perfectly usable folio, use it to retrieve the pfn and order.
All that's needed is a version of folio_file_page that returns a pfn.

Reviewed-by: Michael Roth <michael.roth@amd.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>