netfilter: ip6t_REJECT: added missing icmpv6 codes

RFC 4443 added two new codes values for ICMPv6 type 1:

 5 - Source address failed ingress/egress policy
 6 - Reject route to destination

And RFC 7084 states in L-14 that IPv6 Router MUST send ICMPv6 Destination
Unreachable with code 5 for packets forwarded to it that use an address
from a prefix that has been invalidated.

Codes 5 and 6 are more informative subsets of code 1.

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