do not export kernel's NULL #define to userspace

GCC's NULL is actually __null, which allows detecting some questionable
NULL usage and warn about it.  Moreover each platform/compiler should
have its own stddef.h anyway (which is different from linux/stddef.h).

So there's no good reason to leak kernel's NULL to userspace and
override what the compiler provides.

Signed-off-by: Luboš Luňák <l.lunak@suse.cz>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/stddef.h

@@ -3,15 +3,11 @@
 
 #include <linux/compiler.h>
 
-#undef NULL
-#if defined(__cplusplus)
-#define NULL 0
-#else
-#define NULL ((void *)0)
-#endif
-
 #ifdef __KERNEL__
 
+#undef NULL
+#define NULL ((void *)0)
+
 enum {
 	false	= 0,
 	true	= 1