netfilter: nf_conntrack: fix sysctl memory leak

Message in log because sysctl table was not empty at netns exit
 WARNING: at net/sysctl_net.c:84 sysctl_net_exit+0x2a/0x2c()

Instrumenting showed that the nf_conntrack_timestamp was the entry
that was being created but not cleared.
Signed-off-by: Stephen Hemminger <shemminger@vyatta.com>
Signed-off-by: Patrick McHardy <kaber@trash.net>

net/netfilter/nf_conntrack_core.c

@@ -1294,6 +1294,7 @@ static void nf_conntrack_cleanup_net(struct net *net)
 
 	nf_ct_free_hashtable(net->ct.hash, net->ct.htable_size);
 	nf_conntrack_ecache_fini(net);
+	nf_conntrack_tstamp_fini(net);
 	nf_conntrack_acct_fini(net);
 	nf_conntrack_expect_fini(net);
 	kmem_cache_destroy(net->ct.nf_conntrack_cachep);