IPv6 NAT: Do not drop DNATed 6to4/6rd packets

IPv6 NAT: Do not drop DNATed 6to4/6rd packets

When a router is doing DNAT for 6to4/6rd packets the latest
anti-spoofing commit d6b5dc46 ("ipv6: add anti-spoofing checks for
6to4 and 6rd") will drop them because the IPv6 address embedded does
not match the IPv4 destination. This patch will allow them to pass by
testing if we have an address that matches on 6to4/6rd interface.  I
have been hit by this problem using Fedora and IPV6TO4_IPV4ADDR.
Also, log the dropped packets (with rate limit).

Signed-off-by: Catalin(ux) M. BOIE <catab@embedromix.ro>
Acked-by: Hannes Frederic Sowa <hannes@stressinduktion.org>
Signed-off-by: David S. Miller <davem@davemloft.net>