tcp_cubic: make hystart_ack_delay() aware of BIG TCP

hystart_ack_delay() had the assumption that a TSO packet
would not be bigger than GSO_MAX_SIZE.

This will no longer be true.

We should use sk->sk_gso_max_size instead.

This reduces chances of spurious Hystart ACK train detections.
Signed-off-by: Eric Dumazet <edumazet@google.com>
Acked-by: Alexander Duyck <alexanderduyck@fb.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/tcp_cubic.c

@@ -372,7 +372,7 @@ static void cubictcp_state(struct sock *sk, u8 new_state)
  * We apply another 100% factor because @rate is doubled at this point.
  * We cap the cushion to 1ms.
  */
-static u32 hystart_ack_delay(struct sock *sk)
+static u32 hystart_ack_delay(const struct sock *sk)
 {
 	unsigned long rate;
 
@@ -380,7 +380,7 @@ static u32 hystart_ack_delay(struct sock *sk)
 	if (!rate)
 		return 0;
 	return min_t(u64, USEC_PER_MSEC,
-		     div64_ul((u64)GSO_MAX_SIZE * 4 * USEC_PER_SEC, rate));
+		     div64_ul((u64)sk->sk_gso_max_size * 4 * USEC_PER_SEC, rate));
 }
 
 static void hystart_update(struct sock *sk, u32 delay)