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>
diff --git a/src/vnet/vxlan/vxlan_api.c b/src/vnet/vxlan/vxlan_api.c
index 6c9cbd7..c726c7c 100644
--- a/src/vnet/vxlan/vxlan_api.c
+++ b/src/vnet/vxlan/vxlan_api.c
@@ -59,14 +59,7 @@
 
   VALIDATE_SW_IF_INDEX (mp);
 
-  if (mp->is_ipv6)
-    {
-      /* not yet implemented */
-    }
-  else
-    vnet_feature_enable_disable ("ip4-unicast", "ip4-vxlan-bypass",
-				 sw_if_index, mp->enable, 0, 0);
-
+  vnet_int_vxlan_bypass_mode (sw_if_index, mp->is_ipv6, mp->enable);
   BAD_SW_IF_INDEX_LABEL;
 
   REPLY_MACRO (VL_API_SW_INTERFACE_SET_VXLAN_BYPASS_REPLY);