net: sched: prefer qdisc_is_empty() over direct qlen access

When checking for root qdisc queue length, do not access directly q.qlen.
In the following patches we will move back qlen accounting to per CPU
values for NOLOCK qdiscs.

Instead, prefer the qdisc_is_empty() helper usage.

Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>