ip_tunnel: Provide tunnel metadata API for CONFIG_INET=n

Account for the configuration FIB_RULES=y && INET=n as FIB_RULES can
be selected by IPV6 or DECNET without INET.

Fixes: e703087 ("fib: Add fib rule match on tunnel id")
Fixes: 3093fbe ("route: Per route IP tunnel metadata via lightweight tunnel")
Reported-by: kbuild test robot <fengguang.wu@intel.com>
Signed-off-by: Thomas Graf <tgraf@suug.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/ip_tunnels.h

@@ -314,6 +314,21 @@ static inline int ip_tunnel_collect_metadata(void)
 void ip_tunnel_need_metadata(void);
 void ip_tunnel_unneed_metadata(void);
 
+#else /* CONFIG_INET */
+
+static inline struct ip_tunnel_info *lwt_tun_info(struct lwtunnel_state *lwtstate)
+{
+	return NULL;
+}
+
+static inline void ip_tunnel_need_metadata(void)
+{
+}
+
+static inline void ip_tunnel_unneed_metadata(void)
+{
+}
+
 #endif /* CONFIG_INET */
 
 #endif /* __NET_IP_TUNNELS_H */