VXLAN: Use UDP Tunnel segmention.

Enable TSO for VXLAN devices and use UDP_TUNNEL to offload vxlan
segmentation.

Signed-off-by: Pravin B Shelar <pshelar@nicira.com>
Acked-by: Stephen Hemminger <stephen@networkplumber.org>
Signed-off-by: David S. Miller <davem@davemloft.net>