netfilter: connlimit: factor hlist search into new function

Simplifies followup patch that introduces separate locks for each of
the hash slots.

Reviewed-by: Jesper Dangaard Brouer <brouer@redhat.com>
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>