tcp_cubic: fix spurious HYSTART_DELAY exit upon drop in min RTT

Mirja Kuehlewind reported a bug in Linux TCP CUBIC Hystart, where
Hystart HYSTART_DELAY mechanism can exit Slow Start spuriously on an
ACK when the minimum rtt of a connection goes down. From inspection it
is clear from the existing code that this could happen in an example
like the following:

o The first 8 RTT samples in a round trip are 150ms, resulting in a
  curr_rtt of 150ms and a delay_min of 150ms.

o The 9th RTT sample is 100ms. The curr_rtt does not change after the
  first 8 samples, so curr_rtt remains 150ms. But delay_min can be
  lowered at any time, so delay_min falls to 100ms. The code executes
  the HYSTART_DELAY comparison between curr_rtt of 150ms and delay_min
  of 100ms, and the curr_rtt is declared far enough above delay_min to
  force a (spurious) exit of Slow start.

The fix here is simple: allow every RTT sample in a round trip to
lower the curr_rtt.

Fixes: ae27e98a ("[TCP] CUBIC v2.3")
Reported-by: Mirja Kuehlewind <mirja.kuehlewind@ericsson.com>
Signed-off-by: Neal Cardwell <ncardwell@google.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Acked-by: Soheil Hassas Yeganeh <soheil@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/tcp_cubic.c

@@ -432,10 +432,9 @@ static void hystart_update(struct sock *sk, u32 delay)
 
 	if (hystart_detect & HYSTART_DELAY) {
 		/* obtain the minimum delay of more than sampling packets */
+		if (ca->curr_rtt > delay)
+			ca->curr_rtt = delay;
 		if (ca->sample_cnt < HYSTART_MIN_SAMPLES) {
-			if (ca->curr_rtt > delay)
-				ca->curr_rtt = delay;
-
 			ca->sample_cnt++;
 		} else {
 			if (ca->curr_rtt > ca->delay_min +