ct_dump_stats: detect when stats are not available

Since about 2.6.27 kernel, stats are not enabled by default.
Stats can be enabled using sysctl named

net.netfilter.nf_conntrack_acct

So, do not print zeroes in stats if it's not available.
When not checked, trash may appear in output

lib/netfilter/ct_obj.c

@@ -256,18 +256,29 @@ static void ct_dump_stats(struct nl_object *a, struct nl_dump_params *p)
 	struct nfnl_ct *ct = (struct nfnl_ct *) a;
 	double res;
 	char *unit;
+	uint64_t packets;
+	const char * const names[] = {"rx", "tx"};
+	int i;
 
 	ct_dump_details(a, p);
 
-	nl_dump_line(p, "        # packets      volume\n");
-
-	res = nl_cancel_down_bytes(nfnl_ct_get_bytes(ct, 1), &unit);
-	nl_dump_line(p, "    rx %10llu %7.2f %s\n",
-		nfnl_ct_get_packets(ct, 1), res, unit);
+	if (!nfnl_ct_test_bytes(ct, 0) ||
+	    !nfnl_ct_test_packets(ct, 0) ||
+	    !nfnl_ct_test_bytes(ct, 1) ||
+	    !nfnl_ct_test_packets(ct, 1))
+	    {
+		nl_dump_line(p, "    Statics are not available.\n");
+		nl_dump_line(p, "    Please set sysctl net.netfilter.nf_conntrack_acct = 1\n");
+		nl_dump_line(p, "    (Require kernel 2.6.27)\n");
+		return;
+	    }
 
-	res = nl_cancel_down_bytes(nfnl_ct_get_bytes(ct, 0), &unit);
-	nl_dump_line(p, "    tx %10llu %7.2f %s\n",
-		nfnl_ct_get_packets(ct, 0), res, unit);
+	nl_dump_line(p, "        # packets      volume\n");
+	for (i=0; i<=1; i++) {
+		res = nl_cancel_down_bytes(nfnl_ct_get_bytes(ct, i), &unit);
+		packets = nfnl_ct_get_packets(ct, i);
+		nl_dump_line(p, "    %s %10llu %7.2f %s\n", names[i], packets, res, unit);
+	}
 }
 
 static int ct_compare(struct nl_object *_a, struct nl_object *_b,