br_netfilter: use single forward hook for ip and arp

br_netfilter registers two forward hooks, one for ip and one for arp.

Just use a common function for both and then call the arp/ip helper
as needed.

Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>