tcp_cubic: add SNMP counters to track how effective is Hystart

When deploying FQ pacing, one thing we noticed is that CUBIC Hystart
triggers too soon.

Having SNMP counters to have an idea of how often the various Hystart
methods trigger is useful prior to any modifications.

This patch adds SNMP counters tracking, how many time "ack train" or
"Delay" based Hystart triggers, and cumulative sum of cwnd at the time
Hystart decided to end SS (Slow Start)

myhost:~# nstat -a | grep Hystart
TcpExtTCPHystartTrainDetect     9                  0.0
TcpExtTCPHystartTrainCwnd       20650              0.0
TcpExtTCPHystartDelayDetect     10                 0.0
TcpExtTCPHystartDelayCwnd       360                0.0

->
 Train detection was triggered 9 times, and average cwnd was
 20650/9=2294,
 Delay detection was triggered 10 times and average cwnd was 36

Signed-off-by: Eric Dumazet <edumazet@google.com>
Acked-by: Neal Cardwell <ncardwell@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>