netfilter: nf_dup_netdev: add and use recursion counter

Now that the egress function can be called from egress hook, we need
to avoid recursive calls into the nf_tables traverser, else crash.

Fixes: f87b9464

 ("netfilter: nft_fwd_netdev: Support egress hook")
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>