set fake_rtable's dst to NULL to avoid kernel Oops

bridge: set fake_rtable's dst to NULL to avoid kernel Oops

when bridge is deleted before tap/vif device's delete, kernel may
encounter an oops because of NULL reference to fake_rtable's dst.
Set fake_rtable's dst to NULL before sending packets out can solve
this problem.

v4 reformat, change br_drop_fake_rtable(skb) to {}

v3 enrich commit header

v2 introducing new flag DST_FAKE_RTABLE to dst_entry struct.

[ Use "do { } while (0)" for nop br_drop_fake_rtable()
  implementation -DaveM ]

Acked-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: Peter Huang <peter.huangpeng@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/net/bridge/br_netfilter.c b/net/bridge/br_netfilter.c
index dec4f38..d7f49b6 100644
--- a/net/bridge/br_netfilter.c
+++ b/net/bridge/br_netfilter.c
@@ -156,7 +156,7 @@
 	rt->dst.dev = br->dev;
 	rt->dst.path = &rt->dst;
 	dst_init_metrics(&rt->dst, br_dst_default_metrics, true);
-	rt->dst.flags	= DST_NOXFRM | DST_NOPEER;
+	rt->dst.flags	= DST_NOXFRM | DST_NOPEER | DST_FAKE_RTABLE;
 	rt->dst.ops = &fake_dst_ops;
 }
 
@@ -694,11 +694,7 @@
 				   const struct net_device *out,
 				   int (*okfn)(struct sk_buff *))
 {
-	struct rtable *rt = skb_rtable(skb);
-
-	if (rt && rt == bridge_parent_rtable(in))
-		skb_dst_drop(skb);
-
+	br_drop_fake_rtable(skb);
 	return NF_ACCEPT;
 }