tc: fq: support ce_threshold attribute

tc: fq: support ce_threshold attribute

Kernel commit 48872c11b772 ("net_sched: sch_fq: add dctcp-like marking")
added support for TCA_FQ_CE_THRESHOLD attribute.

This patch adds iproute2 support for it.

It also makes sure fq_print_xstats() can deal with smaller tc_fq_qd_stats
structures given by older kernels.

Usage :

FQATTRS="ce_threshold 4ms"
TXQS=8

for ETH in eth0
do
 tc qd del dev $ETH root 2>/dev/null
 tc qd add dev $ETH root handle 1: mq
 for i in `seq 1 $TXQS`
 do
  tc qd add dev $ETH parent 1:$i fq $FQATTRS
 done
done

Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David Ahern <dsahern@gmail.com>