netfilter: nf_conntrack_bridge: register inet conntrack for bridge

This patch enables IPv4 and IPv6 conntrack from the bridge to deal with
local traffic. Hence, packets that are passed up to the local input path
are confirmed later on from the {ipv4,ipv6}_confirm() hooks.

For packets leaving the IP stack (ie. output path), fragmentation occurs
after the inet postrouting hook. Therefore, the bridge local out and
postrouting bridge hooks see fragments with conntrack objects, which is
inconsistent. In this case, we could defragment again from the bridge
output hook, but this is expensive. The recommended filtering spot for
outgoing locally generated traffic leaving through the bridge interface
is to use the classic IPv4/IPv6 output hook, which comes earlier.

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: David S. Miller <davem@davemloft.net>