ipv6: add anti-spoofing checks for 6to4 and 6rd

This patch adds anti-spoofing checks in sit.c as specified in RFC3964
section 5.2 for 6to4 and RFC5969 section 12 for 6rd. I left out the
checks which could easily be implemented with netfilter.

Specifically this patch adds following logic (based loosely on the
pseudocode in RFC3964 section 5.2):

if prefix (inner_src_v6) == rd6_prefix (2002::/16 is the default)
        and outer_src_v4 != embedded_ipv4 (inner_src_v6)
                drop
if prefix (inner_dst_v6) == rd6_prefix (or 2002::/16 is the default)
        and outer_dst_v4 != embedded_ipv4 (inner_dst_v6)
                drop
accept

To accomplish the specified security checks proposed by above RFCs,
it is still necessary to employ uRPF filters with netfilter. These new
checks only kick in if the employed addresses are within the 2002::/16 or
another range specified by the 6rd-prefix (which defaults to 2002::/16).

Cc: YOSHIFUJI Hideaki <yoshfuji@linux-ipv6.org>
Cc: David Miller <davem@davemloft.net>
Signed-off-by: Hannes Frederic Sowa <hannes@stressinduktion.org>
Signed-off-by: David S. Miller <davem@davemloft.net>