tcp: miscounts due to tcp_fragment pcount reset

It seems that trivial reset of pcount to one was not sufficient
in tcp_retransmit_skb. Multiple counters experience a positive
miscount when skb's pcount gets lowered without the necessary
adjustments (depending on skb's sacked bits which exactly), at
worst a packets_out miscount can crash at RTO if the write queue
is empty!

Triggering this requires mss change, so bidir tcp or mtu probe or
like.
Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@helsinki.fi>
Reported-by: Markus Trippelsdorf <markus@trippelsdorf.de>
Tested-by: Uwe Bugla <uwe.bugla@gmx.de>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/tcp_output.c

@@ -1893,7 +1893,12 @@ int tcp_retransmit_skb(struct sock *sk, struct sk_buff *skb)
 		if (tcp_fragment(sk, skb, cur_mss, cur_mss))
 			return -ENOMEM; /* We'll try again later. */
 	} else {
-		tcp_init_tso_segs(sk, skb, cur_mss);
+		int oldpcount = tcp_skb_pcount(skb);
+
+		if (unlikely(oldpcount > 1)) {
+			tcp_init_tso_segs(sk, skb, cur_mss);
+			tcp_adjust_pcount(sk, skb, oldpcount - tcp_skb_pcount(skb));
+		}
 	}
 
 	tcp_retrans_try_collapse(sk, skb, cur_mss);