net sched actions: bug fix dumping actions directly didnt produce NLMSG_DONE

This refers to commands to direct action access as follows:

sudo tc actions add action drop index 12
sudo tc actions add action pipe index 10

And then dumping them like so:
sudo tc actions ls action gact

iproute2 worked because it depended on absence of TCA_ACT_TAB TLV
as end of message.
This fix has been tested with iproute2 and is backward compatible.
Signed-off-by: Jamal Hadi Salim <jhs@mojatatu.com>
Acked-by: Cong Wang <xiyou.wangcong@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/sched/act_api.c

@@ -1118,7 +1118,7 @@ tc_dump_action(struct sk_buff *skb, struct netlink_callback *cb)
 		nla_nest_end(skb, nest);
 		ret = skb->len;
 	} else
-		nla_nest_cancel(skb, nest);
+		nlmsg_trim(skb, b);
 
 	nlh->nlmsg_len = skb_tail_pointer(skb) - b;
 	if (NETLINK_CB(cb->skb).portid && ret)