VXLAN: Use IP Tunnels tunnel ENC encap API

Use common ecn_encap functions from ip_tunnel module.

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>