ip_tunnel: Call ip_tunnel_core_init() from inet_init()

Convert the module_init() to a invocation from inet_init() since
ip_tunnel_core is part of the INET built-in.

Fixes: 3093fbe7ff ("route: Per route IP tunnel metadata via lightweight tunnel")
Signed-off-by: Thomas Graf <tgraf@suug.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/ip_tunnels.h

@@ -311,6 +311,8 @@ static inline int ip_tunnel_collect_metadata(void)
 	return static_key_false(&ip_tunnel_metadata_cnt);
 }
 
+void __init ip_tunnel_core_init(void);
+
 void ip_tunnel_need_metadata(void);
 void ip_tunnel_unneed_metadata(void);