tcp: fix tcp_ofo_queue() to avoid including too much DUP SACK range

JIRA: https://issues.redhat.com/browse/RHEL-115580

Upstream commit:
commit a041f70
Author: xin.guo <guoxin0309@gmail.com>
Date:   Thu Jun 26 12:34:19 2025 +0000

    tcp: fix tcp_ofo_queue() to avoid including too much DUP SACK range

    If the new coming segment covers more than one skbs in the ofo queue,
    and which seq is equal to rcv_nxt, then the sequence range
    that is duplicated will be sent as DUP SACK, the detail as below,
    in step6, the {501,2001} range is clearly including too much
    DUP SACK range, in violation of RFC 2883 rules.

    1. client > server: Flags [.], seq 501:1001, ack 1325288529, win 20000, length 500
    2. server > client: Flags [.], ack 1, [nop,nop,sack 1 {501:1001}], length 0
    3. client > server: Flags [.], seq 1501:2001, ack 1325288529, win 20000, length 500
    4. server > client: Flags [.], ack 1, [nop,nop,sack 2 {1501:2001} {501:1001}], length 0
    5. client > server: Flags [.], seq 1:2001, ack 1325288529, win 20000, length 2000
    6. server > client: Flags [.], ack 2001, [nop,nop,sack 1 {501:2001}], length 0

    After this fix, the final ACK is as below:

    6. server > client: Flags [.], ack 2001, options [nop,nop,sack 1 {501:1001}], length 0

    [edumazet] added a new packetdrill test in the following patch.

    Fixes: 1da177e ("Linux-2.6.12-rc2")
    Signed-off-by: xin.guo <guoxin0309@gmail.com>
    Signed-off-by: Eric Dumazet <edumazet@google.com>
    Link: https://patch.msgid.link/20250626123420.1933835-2-edumazet@google.com
    Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Signed-off-by: Paolo Abeni <pabeni@redhat.com>

Author: Paolo Abeni

net/ipv4/tcp_input.c

@@ -4974,8 +4974,9 @@ static void tcp_ofo_queue(struct sock *sk)
 
 		if (before(TCP_SKB_CB(skb)->seq, dsack_high)) {
 			__u32 dsack = dsack_high;
+
 			if (before(TCP_SKB_CB(skb)->end_seq, dsack_high))
-				dsack_high = TCP_SKB_CB(skb)->end_seq;
+				dsack = TCP_SKB_CB(skb)->end_seq;
 			tcp_dsack_extend(sk, TCP_SKB_CB(skb)->seq, dsack);
 		}
 		p = rb_next(p);