netfilter: nf_conncount: expose connection list interface

netfilter: nf_conncount: expose connection list interface

commit 5e5cbc7b23eaf13e18652c03efbad5be6995de6a upstream.

This patch provides an interface to maintain the list of connections and
the lookup function to obtain the number of connections in the list.

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
[mfo: backport: refresh context lines and use older symbol/file names:
 - nf_conntrack_count.h: new file, add include guards.
 - nf_conncount.c -> xt_connlimit.c.
   - nf_conncount_rb -> xt_connlimit_rb
   - nf_conncount_tuple -> xt_connlimit_conn
   - conncount_rb_cachep -> connlimit_rb_cachep
   - conncount_conn_cachep -> connlimit_conn_cachep]
Signed-off-by: Mauricio Faria de Oliveira <mfo@canonical.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>