[TCP]: Fix mark_head_lost to ignore R-bit when trying to mark L

This condition (plain R) can arise at least in recovery that
is triggered after tcp_undo_loss. There isn't any reason why
they should not be marked as lost, not marking makes in_flight
estimator to return too large values.
Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@helsinki.fi>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/tcp_input.c

@@ -1987,7 +1987,7 @@ static void tcp_mark_head_lost(struct sock *sk,
 		cnt += tcp_skb_pcount(skb);
 		if (cnt > packets || after(TCP_SKB_CB(skb)->end_seq, high_seq))
 			break;
-		if (!(TCP_SKB_CB(skb)->sacked&TCPCB_TAGBITS)) {
+		if (!(TCP_SKB_CB(skb)->sacked & (TCPCB_SACKED_ACKED|TCPCB_LOST))) {
 			TCP_SKB_CB(skb)->sacked |= TCPCB_LOST;
 			tp->lost_out += tcp_skb_pcount(skb);
 			tcp_verify_retransmit_hint(tp, skb);