netfilter: nft_meta: move pkttype handling to helper

When pkttype is loopback, nft_meta performs guesswork to detect
broad/multicast packets. Place this in a helper, this is hardly a hot path.

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