tcp_bbr: reset long-term bandwidth sampling on loss recovery undo

Fix BBR so that upon notification of a loss recovery undo BBR resets
long-term bandwidth sampling.

Under high reordering, reordering events can be interpreted as loss.
If the reordering and spurious loss estimates are high enough, this
can cause BBR to spuriously estimate that we are seeing loss rates
high enough to trigger long-term bandwidth estimation. To avoid that
problem, this commit resets long-term bandwidth sampling on loss
recovery undo events.

Signed-off-by: Neal Cardwell <ncardwell@google.com>
Reviewed-by: Yuchung Cheng <ycheng@google.com>
Acked-by: Soheil Hassas Yeganeh <soheil@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/tcp_bbr.c

@@ -878,6 +878,7 @@ static u32 bbr_undo_cwnd(struct sock *sk)
 
 	bbr->full_bw = 0;   /* spurious slow-down; reset full pipe detection */
 	bbr->full_bw_cnt = 0;
+	bbr_reset_lt_bw_sampling(sk);
 	return tcp_sk(sk)->snd_cwnd;
 }