tcp_bbr: apply PROBE_RTT cwnd cap even if acked==0

This commit fixes a corner case where TCP BBR would enter PROBE_RTT
mode but not reduce its cwnd. If a TCP receiver ACKed less than one
full segment, the number of delivered/acked packets was 0, so that
bbr_set_cwnd() would short-circuit and exit early, without cutting
cwnd to the value we want for PROBE_RTT.

The fix is to instead make sure that even when 0 full packets are
ACKed, we do apply all the appropriate caps, including the cap that
applies in PROBE_RTT mode.

Fixes: 0f8782ea1497 ("tcp_bbr: add BBR congestion control")
Signed-off-by: Kevin Yang <yyd@google.com>
Signed-off-by: Neal Cardwell <ncardwell@google.com>
Reviewed-by: Yuchung Cheng <ycheng@google.com>
Reviewed-by: Soheil Hassas Yeganeh <soheil@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/tcp_bbr.c

@@ -420,10 +420,10 @@ static void bbr_set_cwnd(struct sock *sk, const struct rate_sample *rs,
 {
 	struct tcp_sock *tp = tcp_sk(sk);
 	struct bbr *bbr = inet_csk_ca(sk);
-	u32 cwnd = 0, target_cwnd = 0;
+	u32 cwnd = tp->snd_cwnd, target_cwnd = 0;
 
 	if (!acked)
-		return;
+		goto done;  /* no packet fully ACKed; just apply caps */
 
 	if (bbr_set_cwnd_to_recover_or_restore(sk, rs, acked, &cwnd))
 		goto done;