mptcp: redundant: avoid multiple retransmissions

Previously redsched_correct_skb_pointers set red_p->skb = NULL when
red_p->skb_start_seq == meta_tp->snd_una.

This issue causes the redundant scheduler to keep sending reinjected
packets again and again.
Replace '!after' with 'before' fixes this issue.

Closes: #474
Fixes: 7835e78 ("mptcp: Fix use-after-free in the redundant scheduler")
Signed-off-by: ytxing <ytxing@mail.ustc.edu.cn>
Signed-off-by: Matthieu Baerts <matthieu.baerts@tessares.net>
(cherry picked from commit 5ffee1d)
Signed-off-by: Matthieu Baerts <matthieu.baerts@tessares.net>
(cherry picked from commit 55abfd0)
Signed-off-by: Matthieu Baerts <matthieu.baerts@tessares.net>

net/mptcp/mptcp_redundant.c

@@ -189,7 +189,7 @@ static void redsched_correct_skb_pointers(struct sock *meta_sk,
 	struct tcp_sock *meta_tp = tcp_sk(meta_sk);
 
 	if (red_p->skb &&
-	    (!after(red_p->skb_start_seq, meta_tp->snd_una) ||
+	    (before(red_p->skb_start_seq, meta_tp->snd_una) ||
 	     after(red_p->skb_end_seq, meta_tp->snd_nxt)))
 		red_p->skb = NULL;
 }