net_sched: always reset qdisc backlog in qdisc_reset()

net_sched: always reset qdisc backlog in qdisc_reset()

BugLink: http://bugs.launchpad.net/bugs/1723145
[ Upstream commit c8e1812960eeae42e2183154927028511c4bc566 ]

SKB stored in qdisc->gso_skb also counted into backlog.

Some qdiscs don't reset backlog to zero in ->reset(),
for example sfq just dequeue and free all queued skb.

Signed-off-by: Konstantin Khlebnikov <khlebnikov@yandex-team.ru>
Fixes: 2ccccf5fb43f ("net_sched: update hierarchical backlog too")
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Seth Forshee <seth.forshee@canonical.com>