netfilter: nft_compat: prepare for indirect info storage

Next patch will make it possible for *info to be stored in
a separate allocation instead of the expr private area.

This removes the 'expr priv area is info blob' assumption
from the match init/destroy/eval functions.

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