neigh: Store hash shift instead of mask.

And mask the hash function result by simply shifting
down the "->hash_shift" most significant bits.

Currently which bits we use is arbitrary since jhash
produces entropy evenly across the whole hash function
result.

But soon we'll be using universal hashing functions,
and in those cases more entropy exists in the higher
bits than the lower bits, because they use multiplies.

Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/neighbour.h

@@ -142,7 +142,7 @@ struct pneigh_entry {
 
 struct neigh_hash_table {
 	struct neighbour __rcu	**hash_buckets;
-	unsigned int		hash_mask;
+	unsigned int		hash_shift;
 	__u32			hash_rnd;
 	struct rcu_head		rcu;
 };