net_sched: fix a use-after-free in tc_ctl_tfilter()

When tcf_destroy() returns true, tp could be already destroyed,
we should not use tp->next after that.

For long term, we probably should move tp list to list_head.

Fixes: 1e052be69d04 ("net_sched: destroy proto tp when all filters are gone")
Cc: Jamal Hadi Salim <jhs@mojatatu.com>
Signed-off-by: Cong Wang <xiyou.wangcong@gmail.com>
Acked-by: Jamal Hadi Salim <jhs@mojatatu.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/sched/cls_api.c

@@ -308,12 +308,11 @@ replay:
 		case RTM_DELTFILTER:
 			err = tp->ops->delete(tp, fh);
 			if (err == 0) {
-				tfilter_notify(net, skb, n, tp, fh, RTM_DELTFILTER);
-				if (tcf_destroy(tp, false)) {
-					struct tcf_proto *next = rtnl_dereference(tp->next);
+				struct tcf_proto *next = rtnl_dereference(tp->next);
 
+				tfilter_notify(net, skb, n, tp, fh, RTM_DELTFILTER);
+				if (tcf_destroy(tp, false))
 					RCU_INIT_POINTER(*back, next);
-				}
 			}
 			goto errout;
 		case RTM_GETTFILTER: