[TCP] MTUprobe: Cleanup send queue check (no need to loop)

The original code has striking complexity to perform a query
which can be reduced to a very simple compare.

FIN seqno may be included to write_seq but it should not make
any significant difference here compared to skb->len which was
used previously. One won't end up there with SYN still queued.

Use of write_seq check guarantees that there's a valid skb in
send_head so I removed the extra check.
Signed-off-by: NIlpo Järvinen <ilpo.jarvinen@helsinki.fi>
Acked-by: NJohn Heffner <jheffner@psc.edu>
Signed-off-by: NHerbert Xu <herbert@gondor.apana.org.au>

net/ipv4/tcp_output.c

@@ -1321,12 +1321,7 @@ static int tcp_mtu_probe(struct sock *sk)
 	}
 
 	/* Have enough data in the send queue to probe? */
-	len = 0;
-	if ((skb = tcp_send_head(sk)) == NULL)
-		return -1;
-	while ((len += skb->len) < size_needed && !tcp_skb_is_last(sk, skb))
-		skb = tcp_write_queue_next(sk, skb);
-	if (len < size_needed)
+	if (tp->write_seq - tp->snd_nxt < size_needed)
 		return -1;
 
 	if (tp->snd_wnd < size_needed)