tcp: force a dst refcount when prequeue packet

Before escaping RCU protected section and adding packet into
prequeue, make sure the dst is refcounted.
Reported-by: NMike Galbraith <bitbucket@online.de>
Signed-off-by: NEric Dumazet <edumazet@google.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

include/net/tcp.h

@@ -1049,6 +1049,7 @@ static inline bool tcp_prequeue(struct sock *sk, struct sk_buff *skb)
 	    skb_queue_len(&tp->ucopy.prequeue) == 0)
 		return false;
 
+	skb_dst_force(skb);
 	__skb_queue_tail(&tp->ucopy.prequeue, skb);
 	tp->ucopy.memory += skb->truesize;
 	if (tp->ucopy.memory > sk->sk_rcvbuf) {