virtio-mem: use page_zonenum() in virtio_mem_fake_offline()

Let's use page_zonenum() instead of zone_idx(page_zone()).

Signed-off-by: David Hildenbrand <david@redhat.com>
Link: https://lore.kernel.org/r/20210602185720.31821-3-david@redhat.com


Signed-off-by: Michael S. Tsirkin <mst@redhat.com>