netfilter: bridge: forward IPv6 fragmented packets

IPv6 fragmented packets are not forwarded on an ethernet bridge
with netfilter ip6_tables loaded. e.g. steps to reproduce

1) create a simple bridge like this

        modprobe br_netfilter
        brctl addbr br0
        brctl addif br0 eth0
        brctl addif br0 eth2
        ifconfig eth0 up
        ifconfig eth2 up
        ifconfig br0 up

2) place a host with an IPv6 address on each side of the bridge

        set IPv6 address on host A:
        ip -6 addr add fd01:2345:6789:1::1/64 dev eth0

        set IPv6 address on host B:
        ip -6 addr add fd01:2345:6789:1::2/64 dev eth0

3) run a simple ping command on host A with packets > MTU

        ping6 -s 4000 fd01:2345:6789:1::2

4) wait some time and run e.g. "ip6tables -t nat -nvL" on the bridge

IPv6 fragmented packets traverse the bridge cleanly until somebody runs.
"ip6tables -t nat -nvL". As soon as it is run (and netfilter modules are
loaded) IPv6 fragmented packets do not traverse the bridge any more (you
see no more responses in ping's output).

After applying this patch IPv6 fragmented packets traverse the bridge
cleanly in above scenario.

Signed-off-by: Bernhard Thaler <bernhard.thaler@wvnet.at>
[pablo@netfilter.org: small changes to br_nf_dev_queue_xmit]
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>