virtio_ring: fix typos in vring_desc_extra

We're actually tracking descriptor address and length instead of the
buffer.

Signed-off-by: Jason Wang <jasowang@redhat.com>
Link: https://lore.kernel.org/r/20211019070152.8236-7-jasowang@redhat.com
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>