netfilter: nft_fwd_netdev: Support egress hook

Allow packet redirection to another interface upon egress.

[lukas: set skb_iif, add commit message, original patch from Pablo. ]
Signed-off-by: Lukas Wunner <lukas@wunner.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>