netfilter: evict stale entries when user reads /proc/net/nf_conntrack

Fabian reports a possible conntrack memory leak (could not reproduce so
far), however, one minor issue can be easily resolved:

> cat /proc/net/nf_conntrack | wc -l = 5
> 4 minutes required to clean up the table.

We should not report those timed-out entries to the user in first place.
And instead of just skipping those timed-out entries while iterating over
the table we can also zap them (we already do this during ctnetlink
walks, but I forgot about the /proc interface).

Fixes: f330a7fd ("netfilter: conntrack: get rid of conntrack timer")
Reported-by: NFabian Frederick <fabf@skynet.be>
Signed-off-by: NFlorian Westphal <fw@strlen.de>
Signed-off-by: NPablo Neira Ayuso <pablo@netfilter.org>

net/netfilter/nf_conntrack_standalone.c

@@ -212,6 +212,11 @@ static int ct_seq_show(struct seq_file *s, void *v)
 	if (unlikely(!atomic_inc_not_zero(&ct->ct_general.use)))
 		return 0;
 
+	if (nf_ct_should_gc(ct)) {
+		nf_ct_kill(ct);
+		goto release;
+	}
+
 	/* we only want to print DIR_ORIGINAL */
 	if (NF_CT_DIRECTION(hash))
 		goto release;