xfrm: interface: use IS_REACHABLE to avoid some compile errors

kernel test robot reported some compile errors:

  ia64-linux-ld: net/xfrm/xfrm_interface.o: in function `xfrmi4_fini':
  net/xfrm/xfrm_interface.c:900: undefined reference to `xfrm4_tunnel_deregister'
  ia64-linux-ld: net/xfrm/xfrm_interface.c:901: undefined reference to `xfrm4_tunnel_deregister'
  ia64-linux-ld: net/xfrm/xfrm_interface.o: in function `xfrmi4_init':
  net/xfrm/xfrm_interface.c:873: undefined reference to `xfrm4_tunnel_register'
  ia64-linux-ld: net/xfrm/xfrm_interface.c:876: undefined reference to `xfrm4_tunnel_register'
  ia64-linux-ld: net/xfrm/xfrm_interface.c:885: undefined reference to `xfrm4_tunnel_deregister'

This happened when set CONFIG_XFRM_INTERFACE=y and CONFIG_INET_TUNNEL=m.
We don't really want xfrm_interface to depend inet_tunnel completely,
but only to disable the tunnel code when inet_tunnel is not seen.

So instead of adding "select INET_TUNNEL" for XFRM_INTERFACE, this patch
is only to change to IS_REACHABLE to avoid these compile error.
Reported-by: kernel test robot <lkp@intel.com>
Fixes: da9bbf05 ("xfrm: interface: support IPIP and IPIP6 tunnels processing with .cb_handler")
Signed-off-by: Xin Long <lucien.xin@gmail.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>

net/xfrm/xfrm_interface.c

@@ -812,7 +812,7 @@ static struct xfrm6_protocol xfrmi_ipcomp6_protocol __read_mostly = {
 	.priority	=	10,
 };
 
-#if IS_ENABLED(CONFIG_INET6_XFRM_TUNNEL)
+#if IS_REACHABLE(CONFIG_INET6_XFRM_TUNNEL)
 static int xfrmi6_rcv_tunnel(struct sk_buff *skb)
 {
 	const xfrm_address_t *saddr;
@@ -863,7 +863,7 @@ static struct xfrm4_protocol xfrmi_ipcomp4_protocol __read_mostly = {
 	.priority	=	10,
 };
 
-#if IS_ENABLED(CONFIG_INET_XFRM_TUNNEL)
+#if IS_REACHABLE(CONFIG_INET_XFRM_TUNNEL)
 static int xfrmi4_rcv_tunnel(struct sk_buff *skb)
 {
 	return xfrm4_rcv_spi(skb, IPPROTO_IPIP, ip_hdr(skb)->saddr);
@@ -897,7 +897,7 @@ static int __init xfrmi4_init(void)
 	err = xfrm4_protocol_register(&xfrmi_ipcomp4_protocol, IPPROTO_COMP);
 	if (err < 0)
 		goto xfrm_proto_comp_failed;
-#if IS_ENABLED(CONFIG_INET_XFRM_TUNNEL)
+#if IS_REACHABLE(CONFIG_INET_XFRM_TUNNEL)
 	err = xfrm4_tunnel_register(&xfrmi_ipip_handler, AF_INET);
 	if (err < 0)
 		goto xfrm_tunnel_ipip_failed;
@@ -908,7 +908,7 @@ static int __init xfrmi4_init(void)
 
 	return 0;
 
-#if IS_ENABLED(CONFIG_INET_XFRM_TUNNEL)
+#if IS_REACHABLE(CONFIG_INET_XFRM_TUNNEL)
 xfrm_tunnel_ipip6_failed:
 	xfrm4_tunnel_deregister(&xfrmi_ipip_handler, AF_INET);
 xfrm_tunnel_ipip_failed:
@@ -924,7 +924,7 @@ static int __init xfrmi4_init(void)
 
 static void xfrmi4_fini(void)
 {
-#if IS_ENABLED(CONFIG_INET_XFRM_TUNNEL)
+#if IS_REACHABLE(CONFIG_INET_XFRM_TUNNEL)
 	xfrm4_tunnel_deregister(&xfrmi_ipip6_handler, AF_INET6);
 	xfrm4_tunnel_deregister(&xfrmi_ipip_handler, AF_INET);
 #endif
@@ -946,7 +946,7 @@ static int __init xfrmi6_init(void)
 	err = xfrm6_protocol_register(&xfrmi_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)
 	err = xfrm6_tunnel_register(&xfrmi_ipv6_handler, AF_INET6);
 	if (err < 0)
 		goto xfrm_tunnel_ipv6_failed;
@@ -957,7 +957,7 @@ static int __init xfrmi6_init(void)
 
 	return 0;
 
-#if IS_ENABLED(CONFIG_INET6_XFRM_TUNNEL)
+#if IS_REACHABLE(CONFIG_INET6_XFRM_TUNNEL)
 xfrm_tunnel_ip6ip_failed:
 	xfrm6_tunnel_deregister(&xfrmi_ipv6_handler, AF_INET6);
 xfrm_tunnel_ipv6_failed:
@@ -973,7 +973,7 @@ static int __init xfrmi6_init(void)
 
 static void xfrmi6_fini(void)
 {
-#if IS_ENABLED(CONFIG_INET6_XFRM_TUNNEL)
+#if IS_REACHABLE(CONFIG_INET6_XFRM_TUNNEL)
 	xfrm6_tunnel_deregister(&xfrmi_ip6ip_handler, AF_INET);
 	xfrm6_tunnel_deregister(&xfrmi_ipv6_handler, AF_INET6);
 #endif