cxgb4: add support for vxlan segmentation offload

add changes to t4_eth_xmit to enable vxlan segmentation
offload support.

Original work by: Santosh Rastapur <santosh@chelsio.com>
Signed-off-by: Ganesh Goudar <ganeshgr@chelsio.com>
Signed-off-by: David S. Miller <davem@davemloft.net>