netfilter: connlimit: improve packet-to-closed-connection logic

Instead of freeing the entry from our list and then adding
it back again in the 'packet to closing connection' case just keep the
matching entry around.  Also drop the found_ct != NULL test as
nf_ct_tuplehash_to_ctrack is just container_of().

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>