extensions: icmp6: added missing icmpv6 dest-unreach codes

extensions: icmp6: added missing icmpv6 dest-unreach codes

https://tools.ietf.org/html/rfc4443 says:

   ICMPv6 Fields:

   Type           1

   Code           0 - No route to destination
                  1 - Communication with destination
                        administratively prohibited
                  2 - Beyond scope of source address
                  3 - Address unreachable
                  4 - Port unreachable
                  5 - Source address failed ingress/egress policy
                  6 - Reject route to destination

Add missing code 2, 5 and 6.

Signed-off-by: Andreas Herz <andi@geekosphere.org>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>