VXLAN tunnel encap forwarding optimization with FIB 2.0

VXLAN tunnel encap forwarding optimization with FIB 2.0

Optimize VXLAN encap node so that vxlan4-encap node is used for IP4
and vxlan6-encap node is used for IP6 underlay. Also stack the VXLAN
encap nodes to the appropriate FIB IP4 or IP6 load-balance node
instead of ip4/ip6-lookup node to save IP lookup operation.

For VXLAN decap node, check VXLAN header FLAGS field for each packet
and remove the code to support decap-next for IP4 or IP6. These decap-
next values were intended for experimentation purposes and not needed
any more since VXLAN-GPE tunnel is supported. The decap-next field is
still kept in API for backward compatibility and its value has no
effect. Decap next for both vxlan4-decap and vxlan6-decap nodes is
always l2-input node.

Change-Id: I8ac95774946549ec403ab691f999df0c006b460f
Signed-off-by: John Lo <loj@cisco.com>