netfilter: ctnetlink: fix incorrect nf_ct_put during hash resize

If nf_conntrack_htable_size was adjusted by the user during the ct
dump operation, we may invoke nf_ct_put twice for the same ct, i.e.
the "last" ct. This will cause the ct will be freed but still linked
in hash buckets.

It's very easy to reproduce the problem by the following commands:
  # while : ; do
  echo $RANDOM > /proc/sys/net/netfilter/nf_conntrack_buckets
  done
  # while : ; do
  conntrack -L
  done
  # iperf -s 127.0.0.1 &
  # iperf -c 127.0.0.1 -P 60 -t 36000

After a while, the system will hang like this:
  NMI watchdog: BUG: soft lockup - CPU#1 stuck for 22s! [bash:20184]
  NMI watchdog: BUG: soft lockup - CPU#0 stuck for 22s! [iperf:20382]
  ...

So at last if we find cb->args[1] is equal to "last", this means hash
resize happened, then we can set cb->args[1] to 0 to fix the above
issue.

Fixes: d205dc40 ("[NETFILTER]: ctnetlink: fix deadlock in table dumping")
Signed-off-by: Liping Zhang <zlpnobody@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

net/netfilter/nf_conntrack_netlink.c

@@ -890,8 +890,13 @@ ctnetlink_dump_table(struct sk_buff *skb, struct netlink_callback *cb)
 	}
 out:
 	local_bh_enable();
-	if (last)
+	if (last) {
+		/* nf ct hash resize happened, now clear the leftover. */
+		if ((struct nf_conn *)cb->args[1] == last)
+			cb->args[1] = 0;
+
 		nf_ct_put(last);
+	}
 
 	while (i) {
 		i--;