[TCP]: Rexmit hint must be cleared instead of setting it

Stupid error from my side. Even though now that I noticed this,
I hoped it would have been an optimization but no, the counter
hint is then incorrect. Thus clearing is necessary for now (I
still suspect though that this path is never executed).
Signed-off-by: NIlpo Järvinen <ilpo.jarvinen@helsinki.fi>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv4/tcp_input.c

@@ -1776,7 +1776,7 @@ static void tcp_verify_retransmit_hint(struct tcp_sock *tp,
 	if ((tp->retransmit_skb_hint != NULL) &&
 	    before(TCP_SKB_CB(skb)->seq,
 	    TCP_SKB_CB(tp->retransmit_skb_hint)->seq))
-		tp->retransmit_skb_hint = skb;
+		tp->retransmit_skb_hint = NULL;
 }
 
 /* Mark head of queue up as lost. */