net_sched: sch_fq: account for schedule/timers drifts

It looks like the following patch can make FQ very precise, even in VM
or stressed hosts. It matters at high pacing rates.

We take into account the difference between the time that was programmed
when last packet was sent, and current time (a drift of tens of usecs is
often observed)

Add an EWMA of the unthrottle latency to help diagnostics.

This latency is the difference between current time and oldest packet in
delayed RB-tree. This accounts for the high resolution timer latency,
but can be different under stress, as fq_check_throttled() can be
opportunistically be called from a dequeue() called after an enqueue()
for a different flow.

Tested:
// Start a 10Gbit flow
$ netperf --google-pacing-rate 1250000000 -H lpaa24 -l 10000 -- -K bbr &

Before patch :
$ sar -n DEV 10 5 | grep eth0 | grep Average
Average:         eth0  17106.04 756876.84   1102.75 1119049.02      0.00      0.00      0.52

After patch :
$ sar -n DEV 10 5 | grep eth0 | grep Average
Average:         eth0  17867.00 800245.90   1151.77 1183172.12      0.00      0.00      0.52

A new iproute2 tc can output the 'unthrottle latency' :

$ tc -s qd sh dev eth0 | grep latency
  0 gc, 0 highprio, 32490767 throttled, 2382 ns latency

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