tc: fq: support low_rate_threshold attribute

tc: fq: support low_rate_threshold attribute

TCA_FQ_LOW_RATE_THRESHOLD sch_fq attribute was added in linux-4.9

Tested:

lpaa5:/tmp# tc -qd add dev eth1 root fq
lpaa5:/tmp# tc -s qd sh dev eth1
qdisc fq 8003: root refcnt 5 limit 10000p flow_limit 1000p buckets 4096 \
 orphan_mask 4095 bands 3 priomap 1 2 2 2 1 2 0 0 1 1 1 1 1 1 1 1 quantum 3648 \
 initial_quantum 18240 low_rate_threshold 550Kbit refill_delay 40.0ms
 Sent 62139 bytes 395 pkt (dropped 0, overlimits 0 requeues 0)
 backlog 0b 0p requeues 0
  116 flows (114 inactive, 0 throttled)
  1 gc, 0 highprio, 0 throttled

lpaa5:/tmp# ./netperf -H lpaa6 -t TCP_RR -l10 -- -q 500000 -r 300,300 -o P99_LATENCY
99th Percentile Latency Microseconds
7081

lpaa5:/tmp# tc qd replace dev eth1 root fq low_rate_threshold 10Mbit
lpaa5:/tmp# ./netperf -H lpaa6 -t TCP_RR -l10 -- -q 500000 -r 300,300 -o P99_LATENCY
99th Percentile Latency Microseconds
858

Signed-off-by: Eric Dumazet <edumazet@google.com>
Acked-by: Soheil Hassas Yeganeh <soheil@google.com>