cxgb4: add data structures to support vxlan

Add data structures and macros to be used in vxlan
offload.

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>