route: Add multipath_hash in flowi_common to make user-define hash

Current fib_multipath_hash_policy can make hash based on the L3 or
L4. But it only work on the outer IP. So a specific tunnel always
has the same hash value. But a specific tunnel may contain so many
inner connections.

This patch provide a generic multipath_hash in floi_common. It can
make a user-define hash which can mix with L3 or L4 hash.

Signed-off-by: wenxu <wenxu@ucloud.cn>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/net/ipv4/ip_gre.c b/net/ipv4/ip_gre.c
index 0f4925a..fd219f7 100644
--- a/net/ipv4/ip_gre.c
+++ b/net/ipv4/ip_gre.c
@@ -578,7 +578,7 @@ static int gre_fill_metadata_dst(struct net_device *dev, struct sk_buff *skb)
 	key = &info->key;
 	ip_tunnel_init_flow(&fl4, IPPROTO_GRE, key->u.ipv4.dst, key->u.ipv4.src,
 			    tunnel_id_to_key32(key->tun_id), key->tos, 0,
-			    skb->mark);
+			    skb->mark, skb_get_hash(skb));
 	rt = ip_route_output_key(dev_net(dev), &fl4);
 	if (IS_ERR(rt))
 		return PTR_ERR(rt);