bridge: Reset IPCB when entering IP stack on NF_FORWARD

Whenever we enter the IP stack proper from bridge netfilter we
need to ensure that the skb is in a form the IP stack expects
it to be in.

The entry point on NF_FORWARD did not meet the requirements of
the IP stack, therefore leading to potential crashes/panics.

This patch fixes the problem.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Acked-by: Stephen Hemminger <shemminger@vyatta.com>
Signed-off-by: David S. Miller <davem@davemloft.net>