[NETFILTER]: bridge: fix double POSTROUTING hook invocation

Packets routed between bridges have the POST_ROUTING hook invoked
twice since bridging mistakes them for bridged packets because
they have skb->nf_bridge set.

Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/net/bridge/br_netfilter.c b/net/bridge/br_netfilter.c
index da22f90..c1757c7 100644
--- a/net/bridge/br_netfilter.c
+++ b/net/bridge/br_netfilter.c
@@ -766,6 +766,9 @@
 	if (!nf_bridge)
 		return NF_ACCEPT;
 
+	if (!(nf_bridge->mask & (BRNF_BRIDGED | BRNF_BRIDGED_DNAT)))
+		return NF_ACCEPT;
+
 	if (!realoutdev)
 		return NF_DROP;