[TCP]: Extend reordering detection to cover CA_Loss partially

This implements more accurately what is stated in sacktag's
overall comment:

  "Both of these heuristics are not used in Loss state, when
   we cannot account for retransmits accurately."

When CA_Loss state is entered, the state changer ensures that
undo_marker is only set if no TCPCB_RETRANS skbs were found,
thus having non-zero undo_marker in CA_Loss basically tells
that the R-bits still accurately reflect the current state
of TCP.
Signed-off-by: NIlpo Järvinen <ilpo.jarvinen@helsinki.fi>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv4/tcp_input.c

@@ -1511,7 +1511,8 @@ tcp_sacktag_write_queue(struct sock *sk, struct sk_buff *ack_skb, u32 prior_snd_
 
 	tcp_verify_left_out(tp);
 
-	if ((reord < tp->fackets_out) && icsk->icsk_ca_state != TCP_CA_Loss &&
+	if ((reord < tp->fackets_out) &&
+	    ((icsk->icsk_ca_state != TCP_CA_Loss) || tp->undo_marker) &&
 	    (!tp->frto_highmark || after(tp->snd_una, tp->frto_highmark)))
 		tcp_update_reordering(sk, tp->fackets_out - reord, 0);