fib: Add fib rule match on tunnel id

This add the ability to select a routing table based on the tunnel
id which allows to maintain separate routing tables for each virtual
tunnel network.

ip rule add from all tunnel-id 100 lookup 100
ip rule add from all tunnel-id 200 lookup 200

A new static key controls the collection of metadata at tunnel level
upon demand.

Signed-off-by: Thomas Graf <tgraf@suug.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/fib_rules.h

@@ -19,6 +19,7 @@ struct fib_rule {
 	u8			action;
 	/* 3 bytes hole, try to use */
 	u32			target;
+	__be64			tun_id;
 	struct fib_rule __rcu	*ctarget;
 	struct net		*fr_net;