linux/virtio_net.h: Support USO offload in vnet header.

Now, it's possible to convert USO vnet packets from/to skb.
Added support for GSO_UDP_L4 offload.

Signed-off-by: Andrew Melnychenko <andrew@daynix.com>
Signed-off-by: David S. Miller <davem@davemloft.net>