netfilter: nf_tables: only allow in/output for arp packets

arp packets cannot be forwarded.

They can be bridged, but then they can be filtered using
either ebtables or nftables bridge family.

The bridge netfilter exposes a "call-arptables" switch which
pushes packets into arptables, but lets not expose this for nftables, so better
close this asap.

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