bridge: fix forwarding of IPv6

The commit 6b1e960f
    bridge: Reset IPCB when entering IP stack on NF_FORWARD
broke forwarding of IPV6 packets in bridge because it would
call bp_parse_ip_options with an IPV6 packet.

Reported-by: Noah Meyerhans <noahm@debian.org>
Signed-off-by: Stephen Hemminger <shemminger@vyatta.com>
Reviewed-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>