ipv4: Make neigh lookup keys for loopback/point-to-point devices be INADDR_ANY

Map all lookup neigh keys to INADDR_ANY for loopback/point-to-point devices
to avoid making an entry for every remote ip the device needs to talk to.

This used the be the old behavior but became broken in a263b3093641f
(ipv4: Make neigh lookups directly in output packet path) and later removed
in 0bb4087cbec0 (ipv4: Fix neigh lookup keying over loopback/point-to-point
devices) because it was broken.

Signed-off-by: Jim Westfall <jwestfall@surrealistic.net>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/include/net/arp.h b/include/net/arp.h
index dc8cd47..977aabf 100644
--- a/include/net/arp.h
+++ b/include/net/arp.h
@@ -20,6 +20,9 @@ static inline u32 arp_hashfn(const void *pkey, const struct net_device *dev, u32
 
 static inline struct neighbour *__ipv4_neigh_lookup_noref(struct net_device *dev, u32 key)
 {
+	if (dev->flags & (IFF_LOOPBACK | IFF_POINTOPOINT))
+		key = INADDR_ANY;
+
 	return ___neigh_lookup_noref(&arp_tbl, neigh_key_eq32, arp_hashfn, &key, dev);
 }