virtio_ring: rename vring_desc_extra_packed

Rename vring_desc_extra_packed to vring_desc_extra since the structure
are pretty generic which could be reused by split virtqueue as well.

Signed-off-by: Jason Wang <jasowang@redhat.com>
Link: https://lore.kernel.org/r/20210604055350.58753-3-jasowang@redhat.com


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