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>

include/net/dst.h

@@ -59,6 +59,7 @@ struct dst_entry {
 #define DST_NOCACHE		0x0010
 #define DST_NOCOUNT		0x0020
 #define DST_NOPEER		0x0040
+#define DST_FAKE_RTABLE		0x0080
 
 	short			error;
 	short			obsolete;