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>
diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
index c0b574a..3d932e6 100644
--- a/include/linux/skbuff.h
+++ b/include/linux/skbuff.h
@@ -170,12 +170,14 @@
 		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