netfilter: nfnetlink_log: fix mac address for 6in4 tunnels

For tunnelled ipv6in4 packets, the LOG target (xt_LOG.c) adjusts
the start of the mac field to start at the ethernet header instead
of the ipv4 header for the tunnel. This patch conforms what is
passed by the NFLOG target through nfnetlink to what the LOG target
does. Code borrowed from xt_LOG.c.

Signed-off-by: Bob Hockney <bhockney@ix.netcom.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>