ip6_vti: use IS_REACHABLE to avoid some compile errors

Naresh reported some compile errors:

  arm build failed due this error on linux-next 20200713 and  20200713
  net/ipv6/ip6_vti.o: In function `vti6_rcv_tunnel':
  ip6_vti.c:(.text+0x1d20): undefined reference to `xfrm6_tunnel_spi_lookup'

This happened when set CONFIG_IPV6_VTI=y and CONFIG_INET6_TUNNEL=m.
We don't really want ip6_vti to depend inet6_tunnel completely, but
only to disable the tunnel code when inet6_tunnel is not seen.

So instead of adding "select INET6_TUNNEL" for IPV6_VTI, this patch
is only to change to IS_REACHABLE to avoid these compile error.
Reported-by: Naresh Kamboju <naresh.kamboju@linaro.org>
Fixes: 08622869 ("ip6_vti: support IP6IP6 tunnel processing with .cb_handler")
Signed-off-by: Xin Long <lucien.xin@gmail.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>

net/ipv6/ip6_vti.c

@@ -1218,7 +1218,7 @@ static struct xfrm6_protocol vti_ipcomp6_protocol __read_mostly = {
 	.priority	=	100,
 };
 
-#if IS_ENABLED(CONFIG_INET6_XFRM_TUNNEL)
+#if IS_REACHABLE(CONFIG_INET6_XFRM_TUNNEL)
 static int vti6_rcv_tunnel(struct sk_buff *skb)
 {
 	const xfrm_address_t *saddr;
@@ -1270,7 +1270,7 @@ static int __init vti6_tunnel_init(void)
 	err = xfrm6_protocol_register(&vti_ipcomp6_protocol, IPPROTO_COMP);
 	if (err < 0)
 		goto xfrm_proto_comp_failed;
-#if IS_ENABLED(CONFIG_INET6_XFRM_TUNNEL)
+#if IS_REACHABLE(CONFIG_INET6_XFRM_TUNNEL)
 	msg = "ipv6 tunnel";
 	err = xfrm6_tunnel_register(&vti_ipv6_handler, AF_INET6);
 	if (err < 0)
@@ -1288,7 +1288,7 @@ static int __init vti6_tunnel_init(void)
 	return 0;
 
 rtnl_link_failed:
-#if IS_ENABLED(CONFIG_INET6_XFRM_TUNNEL)
+#if IS_REACHABLE(CONFIG_INET6_XFRM_TUNNEL)
 	err = xfrm6_tunnel_deregister(&vti_ip6ip_handler, AF_INET);
 vti_tunnel_ip6ip_failed:
 	err = xfrm6_tunnel_deregister(&vti_ipv6_handler, AF_INET6);
@@ -1312,7 +1312,7 @@ static int __init vti6_tunnel_init(void)
 static void __exit vti6_tunnel_cleanup(void)
 {
 	rtnl_link_unregister(&vti6_link_ops);
-#if IS_ENABLED(CONFIG_INET6_XFRM_TUNNEL)
+#if IS_REACHABLE(CONFIG_INET6_XFRM_TUNNEL)
 	xfrm6_tunnel_deregister(&vti_ip6ip_handler, AF_INET);
 	xfrm6_tunnel_deregister(&vti_ipv6_handler, AF_INET6);
 #endif