ipv6: Use a 31-bit multipath hash

The hash thresholds assigned to IPv6 nexthops are in the range of
[-1, 2^31 - 1], where a negative value is assigned to nexthops that
should not be considered during multipath selection.

Therefore, in a similar fashion to IPv4, we need to use the upper
31-bits of the multipath hash for multipath selection.
Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Acked-by: David Ahern <dsahern@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv6/route.c

@@ -1833,10 +1833,10 @@ u32 rt6_multipath_hash(const struct flowi6 *fl6, const struct sk_buff *skb)
 
 	if (skb) {
 		ip6_multipath_l3_keys(skb, &hash_keys);
-		return flow_hash_from_keys(&hash_keys);
+		return flow_hash_from_keys(&hash_keys) >> 1;
 	}
 
-	return get_hash_from_flowi6(fl6);
+	return get_hash_from_flowi6(fl6) >> 1;
 }
 
 void ip6_route_input(struct sk_buff *skb)