net/sched: sch_api: add helper for tc qdisc walker stats dump

The walk implementation of most qdisc class modules is basically the
same. That is, the values of count and skip are checked first. If
count is greater than or equal to skip, the registered fn function is
executed. Otherwise, increase the value of count. So we can reconstruct
them.

Signed-off-by: Zhengchao Shao <shaozhengchao@huawei.com>
Acked-by: Toke Høiland-Jørgensen <toke@redhat.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>