ipv4: Fix neigh lookup keying over loopback/point-to-point devices.

We were using a special key "0" for all loopback and point-to-point
device neigh lookups under ipv4, but we wouldn't use that special
key for the neigh creation.

So basically we'd make a new neigh at each and every lookup :-)

This special case to use only one neigh for these device types
is of dubious value, so just remove it entirely.
Reported-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/arp.h

@@ -21,9 +21,6 @@ static inline struct neighbour *__ipv4_neigh_lookup_noref(struct net_device *dev
 	struct neighbour *n;
 	u32 hash_val;
 
-	if (dev->flags & (IFF_LOOPBACK | IFF_POINTOPOINT))
-		key = 0;
-
 	hash_val = arp_hashfn(key, dev, nht->hash_rnd[0]) >> (32 - nht->hash_shift);
 	for (n = rcu_dereference_bh(nht->hash_buckets[hash_val]);
 	     n != NULL;