net: sched: fix dump qlen for sch_mq/sch_mqprio with NOLOCK subqueues

sch->q.len hasn't been set if the subqueue is a NOLOCK qdisc
 in mq_dump() and mqprio_dump().

Fixes: ce679e8d ("net: sched: add support for TCQ_F_NOLOCK subqueues to sch_mqprio")
Signed-off-by: Dust Li <dust.li@linux.alibaba.com>
Signed-off-by: Tony Lu <tonylu@linux.alibaba.com>
Signed-off-by: David S. Miller <davem@davemloft.net>