[NET_SCHED] sch_prio: class statistics printing enabled

This patch adds a dump_stats callback to enable
printing of basic statistics of prio classes.
(With help of Patrick McHardy).

Signed-off-by: Jarek Poplawski <jarkao2@o2.pl>
Acked-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>