net-tcp_bbr: v2: don't assume prior_cwnd was set entering CA_Loss

Fix WARN_ON_ONCE() warnings that were firing and pointing to a
bbr->prior_cwnd of 0 when exiting CA_Loss and transitioning to
CA_Open.

The issue was that tcp_simple_retransmit() calls:

  tcp_set_ca_state(sk, TCP_CA_Loss);

without first calling icsk_ca_ops->ssthresh(sk) (because
tcp_simple_retransmit() is dealing with losses due to MTU issues and
not congestion). The lack of this callback means that BBR did not get
a chance to set bbr->prior_cwnd, and thus upon exiting CA_Loss in such
cases the WARN_ON_ONCE() would fire due to a zero bbr->prior_cwnd.

This commit removes that warning, since a bbr->prior_cwnd of 0 is a
valid situation in this state transition.

For setting inflight_lo upon entering CA_Loss, to avoid setting an
inflight_lo of 0 in this case, this commit switches to taking the max
of cwnd and prior_cwnd. We plan to remove that line of code when we
switch to cautious (PRR-style) recovery, so that awkwardness will go
away.

Change-Id: I575dce871c2f20e91e3e9449e1706f42a07b8118

net/ipv4/tcp_bbr2.c

@@ -2617,15 +2617,12 @@ static void bbr2_set_state(struct sock *sk, u8 new_state)
 			/* bbr_adapt_lower_bounds() needs cwnd before
 			 * we suffered an RTO, to update inflight_lo:
 			 */
-			WARN_ON_ONCE(bbr->prior_cwnd == 0);
-			WARN_ON_ONCE(bbr->prior_cwnd == ~0U);
-			bbr->inflight_lo = bbr->prior_cwnd;
+			bbr->inflight_lo =
+				max(tp->snd_cwnd, bbr->prior_cwnd);
 		}
 		bbr_debug(sk, 0, &rs, &ctx);
 	} else if (bbr->prev_ca_state == TCP_CA_Loss &&
 		   new_state != TCP_CA_Loss) {
-		WARN_ON_ONCE(bbr->prior_cwnd == 0);
-		WARN_ON_ONCE(bbr->prior_cwnd == ~0U);
 		tp->snd_cwnd = max(tp->snd_cwnd, bbr->prior_cwnd);
 		bbr->try_fast_path = 0; /* bound cwnd using latest model */
 	}