virtio-net: add DATA_VALID flag

Add DATA_VALID flag from the Linux header, to
keep us in sync with that.
Signed-off-by: NMichael S. Tsirkin <mst@redhat.com>

hw/virtio-net.h

@@ -79,6 +79,7 @@ struct virtio_net_config
 struct virtio_net_hdr
 {
 #define VIRTIO_NET_HDR_F_NEEDS_CSUM     1       // Use csum_start, csum_offset
+#define VIRTIO_NET_HDR_F_DATA_VALID	2	// Csum is valid
     uint8_t flags;
 #define VIRTIO_NET_HDR_GSO_NONE         0       // Not a GSO frame
 #define VIRTIO_NET_HDR_GSO_TCPV4        1       // GSO frame, IPv4 TCP (TSO)