tcp: fix tcp_cleanup_rbuf() for tcp_read_skb()

tcp_cleanup_rbuf() retrieves the skb from sk_receive_queue, it
assumes the skb is not yet dequeued. This is no longer true for
tcp_read_skb() case where we dequeue the skb first.

Fix this by introducing a helper __tcp_cleanup_rbuf() which does
not require any skb and calling it in tcp_read_skb().

Fixes: 04919bed948d ("tcp: Introduce tcp_read_skb()")
Cc: Eric Dumazet <[e-mail]>
Cc: John Fastabend <[e-mail]>
Cc: Jakub Sitnicki <[e-mail]>
Signed-off-by: Cong Wang <[e-mail]>
Signed-off-by: Jakub Kicinski <[e-mail]>

net/ipv4/tcp.c

···
  * calculation of whether or not we must ACK for the sake of
  * a window update.
  */
-void tcp_cleanup_rbuf(struct sock *sk, int copied)
+static void __tcp_cleanup_rbuf(struct sock *sk, int copied)
 {
 	struct tcp_sock *tp = tcp_sk(sk);
 	bool time_to_ack = false;
-
-	struct sk_buff *skb = skb_peek(&sk->sk_receive_queue);
-
-	WARN(skb && !before(tp->copied_seq, TCP_SKB_CB(skb)->end_seq),
-	     "cleanup rbuf bug: copied %X seq %X rcvnxt %X\n",
-	     tp->copied_seq, TCP_SKB_CB(skb)->end_seq, tp->rcv_nxt);
 
 	if (inet_csk_ack_scheduled(sk)) {
 		const struct inet_connection_sock *icsk = inet_csk(sk);
···
 	}
 	if (time_to_ack)
 		tcp_send_ack(sk);
+}
+
+void tcp_cleanup_rbuf(struct sock *sk, int copied)
+{
+	struct sk_buff *skb = skb_peek(&sk->sk_receive_queue);
+	struct tcp_sock *tp = tcp_sk(sk);
+
+	WARN(skb && !before(tp->copied_seq, TCP_SKB_CB(skb)->end_seq),
+	     "cleanup rbuf bug: copied %X seq %X rcvnxt %X\n",
+	     tp->copied_seq, TCP_SKB_CB(skb)->end_seq, tp->rcv_nxt);
+	__tcp_cleanup_rbuf(sk, copied);
 }
 
 static void tcp_eat_recv_skb(struct sock *sk, struct sk_buff *skb)
···
 		copied += used;
 
 		if (TCP_SKB_CB(skb)->tcp_flags & TCPHDR_FIN) {
-			consume_skb(skb);
 			++seq;
 			break;
 		}
-		consume_skb(skb);
 		break;
 	}
+	consume_skb(skb);
 	WRITE_ONCE(tp->copied_seq, seq);
 
 	tcp_rcv_space_adjust(sk);
 
 	/* Clean up data we have read: This will do ACK frames. */
 	if (copied > 0)
-		tcp_cleanup_rbuf(sk, copied);
+		__tcp_cleanup_rbuf(sk, copied);
 
 	return copied;
 }