bridge: Save frag_max_size between PRE_ROUTING and POST_ROUTING

As we may defragment the packet in IPv4 PRE_ROUTING and refragment
it after POST_ROUTING we should save the value of frag_max_size.

This is still very wrong as the bridge is supposed to leave the
packets intact, meaning that the right thing to do is to use the
original frag_list for fragmentation.

Unfortunately we don't currently guarantee that the frag_list is
left untouched throughout netfilter so until this changes this is
the best we can do.

There is also a spot in FORWARD where it appears that we can
forward a packet without going through fragmentation, mark it
so that we can fix it later.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>