netfilter: nf_ct_ipv4: handle invalid IPv4 and IPv6 packets consistently

IPv6 conntrack marked invalid packets as INVALID and let the user
drop those by an explicit rule, while IPv4 conntrack dropped such
packets itself.

IPv4 conntrack is changed so that it marks INVALID packets and let
the user to drop them.

Signed-off-by: Jozsef Kadlecsik <kadlec@blackhole.kfki.hu>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>