Add vxlan-bypass feature to IP6 forwarding path

Add vxlan-bypass feature to IP6 forwarding path

Add vxlan-bypass feature which can be enabled on the IP6 underlay
interface which receive VXLAN packets to accelerate VXLAN decap
processing. The CLI to enable/disable it is:
    set interface ip6 vxlan-bypass <interface> [del]
The vxlan-bypass feature is already supported on the IP4 underlay
interface. The CLI to enable/disable it is:
    set interface ip vxlan-bypass <interface> [del]

Move vxlan-bypass API/CLI support code from decap.c to vxlan.c.
Also fixed two issues in the VXLAN decap path in the vxlan-input node:
1. Add verification of VXLAN packet FIB index with the encap-vrf-id
   of the VXLAN tunnel.
2. Fix checking of VXLANoIPv6 packet mcast DIP against that of the
   IP6 mcast VXLAN tunnel.

Change-Id: I2bad4074a468c48fbb8bb5ac64f6437190756ed2
Signed-off-by: John Lo <loj@cisco.com>