virtio-net: restore VIRTIO_HDR_F_DATA_VALID on receiving

Commit 501db511 ("virtio: don't set VIRTIO_NET_HDR_F_DATA_VALID on
xmit") in fact disables VIRTIO_HDR_F_DATA_VALID on receiving path too,
fixing this by adding a hint (has_data_valid) and set it only on the
receiving path.

Cc: Rolf Neugebauer <rolf.neugebauer@docker.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>
Acked-by: Rolf Neugebauer <rolf.neugebauer@docker.com>
Signed-off-by: David S. Miller <davem@davemloft.net>