[TCP] FRTO: Clear frto_highmark only after process_frto that uses it

I broke this in commit 3de96471bd7fb76406e975ef6387abe3a0698149:

	[TCP]: Wrap-safed reordering detection FRTO check

tcp_process_frto should always see a valid frto_highmark. An invalid
frto_highmark (zero) is very likely what ultimately caused a seqno
compare in tcp_frto_enter_loss to do the wrong leading to the LOST-bit
leak.

Having LOST-bits integry ensured like done after commit
23aeeec365dcf8bc87fae44c533e50d0bb4f23cc:

	[TCP] FRTO: Plug potential LOST-bit leak

won't hurt. It may still be useful in some other, possibly legimate,
scenario.

Reported by Chazarain Guillaume <guichaz@yahoo.fr>.

Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@helsinki.fi>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/tcp_input.c

@@ -3113,11 +3113,11 @@ static int tcp_ack(struct sock *sk, struct sk_buff *skb, int flag)
 	/* See if we can take anything off of the retransmit queue. */
 	flag |= tcp_clean_rtx_queue(sk, &seq_rtt, prior_fackets);
 
+	if (tp->frto_counter)
+		frto_cwnd = tcp_process_frto(sk, flag);
 	/* Guarantee sacktag reordering detection against wrap-arounds */
 	if (before(tp->frto_highmark, tp->snd_una))
 		tp->frto_highmark = 0;
-	if (tp->frto_counter)
-		frto_cwnd = tcp_process_frto(sk, flag);
 
 	if (tcp_ack_is_dubious(sk, flag)) {
 		/* Advance CWND, if state allows this. */