[PKT_SCHED]: Add dump_stats qdisc op

Adds a new qdisc operation dump_stats which qdiscs can use
to dump statistics. The op is invoked between gnet_stats_start_copy
and gnet_stats_finish_copy and therefore stats_lock is already
held. This is required to ensure proper locking throughout the
whole statistic dumping procedure.
Signed-off-by: Thomas Graf <tgraf@suug.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/pkt_sched.h

@@ -64,6 +64,7 @@ struct Qdisc_ops
 	int			(*change)(struct Qdisc *, struct rtattr *arg);
 
 	int			(*dump)(struct Qdisc *, struct sk_buff *);
+	int			(*dump_stats)(struct Qdisc *, struct gnet_dump *);
 
 	struct module		*owner;
 };

net/sched/sch_api.c

@@ -767,6 +767,9 @@ static int tc_fill_qdisc(struct sk_buff *skb, struct Qdisc *q, u32 clid,
 			TCA_XSTATS, q->stats_lock, &d) < 0)
 		goto rtattr_failure;
 
+	if (q->ops->dump_stats && q->ops->dump_stats(q, &d) < 0)
+		goto rtattr_failure;
+
 	if (gnet_stats_copy_basic(&d, &q->bstats) < 0 ||
 #ifdef CONFIG_NET_ESTIMATOR
 	    gnet_stats_copy_rate_est(&d, &q->rate_est) < 0 ||