Make ip csum configurable in vlib buffer functions

Also fixes csum computation for lisp control plane 4o6 encapsulated control
messages.

Change-Id: I991e0b5c0d16dc51e0b5bdc79e1d752270b34765
Signed-off-by: Florin Coras <fcoras@cisco.com>
diff --git a/src/vnet/ip/ip6.h b/src/vnet/ip/ip6.h
index cf52994..fa92272 100644
--- a/src/vnet/ip/ip6.h
+++ b/src/vnet/ip/ip6.h
@@ -576,6 +576,7 @@
 	       sizeof (ip6h->src_address));
   clib_memcpy (ip6h->dst_address.as_u8, dst->as_u8,
 	       sizeof (ip6h->src_address));
+  b->flags |= VNET_BUFFER_F_IS_IP6;
 
   return ip6h;
 }