netfilter: bridge: neigh_head and physoutdev can't be used at same time

The neigh_header is only needed when we detect DNAT after prerouting
and neigh cache didn't have a mac address for us.

The output port has not been chosen yet so we can re-use the storage
area, bringing struct size down to 32 bytes on x86_64.
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

include/linux/skbuff.h

@@ -170,12 +170,14 @@ struct nf_bridge_info {
 		BRNF_PROTO_UNCHANGED,
 		BRNF_PROTO_8021Q,
 		BRNF_PROTO_PPPOE
-	} orig_proto;
+	} orig_proto:8;
 	bool			pkt_otherhost;
 	unsigned int		mask;
 	struct net_device	*physindev;
-	struct net_device	*physoutdev;
-	char			neigh_header[8];
+	union {
+		struct net_device *physoutdev;
+		char neigh_header[8];
+	};
 };
 #endif
 

net/bridge/br_netfilter.c

@@ -973,6 +973,8 @@ static void br_nf_pre_routing_finish_bridge_slow(struct sk_buff *skb)
 				       nf_bridge->neigh_header,
 				       ETH_HLEN - ETH_ALEN);
 	skb->dev = nf_bridge->physindev;
+
+	nf_bridge->physoutdev = NULL;
 	br_handle_frame_finish(NULL, skb);
 }