netfilter: bridge: add connection tracking system

This patch adds basic connection tracking support for the bridge,
including initial IPv4 support.

This patch register two hooks to deal with the bridge forwarding path,
one from the bridge prerouting hook to call nf_conntrack_in(); and
another from the bridge postrouting hook to confirm the entry.

The conntrack bridge prerouting hook defragments packets before passing
them to nf_conntrack_in() to look up for an existing entry, otherwise a
new entry is allocated and it is attached to the skbuff. The conntrack
bridge postrouting hook confirms new conntrack entries, ie. if this is
the first packet seen, then it adds the entry to the hashtable and (if
needed) it refragments the skbuff into the original fragments, leaving
the geometry as is if possible. Exceptions are linearized skbuffs, eg.
skbuffs that are passed up to nfqueue and conntrack helpers, as well as
cloned skbuff for the local delivery (eg. tcpdump), also in case of
bridge port flooding (cloned skbuff too).

The packet defragmentation is done through the ip_defrag() call.  This
forces us to save the bridge control buffer, reset the IP control buffer
area and then restore it after call. This function also bumps the IP
fragmentation statistics, it would be probably desiderable to have
independent statistics for the bridge defragmentation/refragmentation.
The maximum fragment length is stored in the control buffer and it is
used to refragment the skbuff from the postrouting path.

The new fraglist splitter and fragment transformer APIs are used to
implement the bridge refragmentation code. The br_ip_fragment() function
drops the packet in case the maximum fragment size seen is larger than
the output port MTU.

This patchset follows the principle that conntrack should not drop
packets, so users can do it through policy via invalid state matching.

Like br_netfilter, there is no refragmentation for packets that are
passed up for local delivery, ie. prerouting -> input path. There are
calls to nf_reset() already in several spots in the stack since time ago
already, eg. af_packet, that show that skbuff fraglist handling from the
netif_rx path is supported already.

The helpers are called from the postrouting hook, before confirmation,
from there we may see packet floods to bridge ports. Then, although
unlikely, this may result in exercising the helpers many times for each
clone. It would be good to explore how to pass all the packets in a list
to the conntrack hook to do this handle only once for this case.

Thanks to Florian Westphal for handing me over an initial patchset
version to add support for conntrack bridge.

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