netfilter: bridge: Drops IPv6 packets if IPv6 module is not loaded

A kernel panic can happen if a host has disabled IPv6 on boot and have to
process guest packets (coming from a bridge) using it's ip6tables.

IPv6 packets need to be dropped if the IPv6 module is not loaded, and the
host ip6tables will be used.

Signed-off-by: Leonardo Bras <leonardo@linux.ibm.com>
Acked-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>