Mcast rewrite optimisations

hard code the address mask offsets. This are protocol specific and only used on ethernet when used at all.

Change-Id: Ib1f6f33682f53254ffbb5a241a1583e65420e0c7
Signed-off-by: Neale Ranns <nranns@cisco.com>
diff --git a/src/vnet/ip/ip6_neighbor.c b/src/vnet/ip/ip6_neighbor.c
index fee4356..8e44402 100644
--- a/src/vnet/ip/ip6_neighbor.c
+++ b/src/vnet/ip/ip6_neighbor.c
@@ -672,10 +672,10 @@
 	 * Complete the remaining fields of the adj's rewrite to direct the
 	 * complete of the rewrite at switch time by copying in the IP
 	 * dst address's bytes.
-	 * Ofset is 2 bytes into the desintation address. And we write 4 bytes.
+	 * Ofset is 2 bytes into the desintation address.
 	 */
 	offset = vec_len (rewrite) - 2;
-	adj_mcast_update_rewrite (ai, rewrite, offset, 0xffffffff);
+	adj_mcast_update_rewrite (ai, rewrite, offset);
 
 	break;
       }