virtio-net: don't let virtio core to validate used length

For RX virtuqueue, the used length is validated in all the three paths
(big, small and mergeable). For control vq, we never tries to use used
length. So this patch forbids the core to validate the used length.

Signed-off-by: Jason Wang <jasowang@redhat.com>
Link: https://lore.kernel.org/r/20211027022107.14357-3-jasowang@redhat.com
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>