netfilter: nft_ct: add zone id get support

Just like with counters the direction attribute is optional.
We set priv->dir to MAX unconditionally to avoid duplicating the assignment
for all keys with optional direction.

For keys where direction is mandatory, existing code already returns
an error.

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