netfilter: nf_nat_snmp_basic: replace ctinfo with dir.

The snmp_translate() receives ctinfo data to get dir value only.
because of caller already has dir value, we just replace ctinfo with dir.

Signed-off-by: Taehee Yoo <ap420073@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>