net: Fix userland breakage wrt. linux/if_tunnel.h

Reported by Andrew Walrond <andrew@walrond.org>

Changeset c19e654d
("gre: Add netlink interface") added an include
of linux/ip.h to linux/if_tunnel.h

We can't really let that get exposed to userspace
because this conflicts with types defined in netinet/ip.h
which userland is almost certainly going to have included
either explicitly or implicitly.

So guard this include with a __KERNEL__ ifdef.
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/Kbuild

@@ -89,7 +89,6 @@ header-y += if_ppp.h
 header-y += if_slip.h
 header-y += if_strip.h
 header-y += if_tun.h
-header-y += if_tunnel.h
 header-y += in_route.h
 header-y += ioctl.h
 header-y += ip6_tunnel.h
@@ -235,6 +234,7 @@ unifdef-y += if_phonet.h
 unifdef-y += if_pppol2tp.h
 unifdef-y += if_pppox.h
 unifdef-y += if_tr.h
+unifdef-y += if_tunnel.h
 unifdef-y += if_vlan.h
 unifdef-y += igmp.h
 unifdef-y += inet_diag.h

include/linux/if_tunnel.h

@@ -2,7 +2,10 @@
 #define _IF_TUNNEL_H_
 
 #include <linux/types.h>
+
+#ifdef __KERNEL__
 #include <linux/ip.h>
+#endif
 
 #define SIOCGETTUNNEL   (SIOCDEVPRIVATE + 0)
 #define SIOCADDTUNNEL   (SIOCDEVPRIVATE + 1)