linux/kconfig.h: replace IF_ENABLED() with PTR_IF() in <linux/kernel.h>

<linux/kconfig.h> is included from all the kernel-space source files,
including C, assembly, linker scripts. It is intended to contain a
minimal set of macros to evaluate CONFIG options.

IF_ENABLED() is an intruder here because (x ? y : z) is C code, which
should not be included from assembly files or linker scripts.

Also, <linux/kconfig.h> is no longer self-contained because NULL is
defined in <linux/stddef.h>.

Move IF_ENABLED() out to <linux/kernel.h> as PTR_IF(). PTF_IF()
takes the general boolean expression instead of a CONFIG option
so that it fits better in <linux/kernel.h>.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Kees Cook <keescook@chromium.org>

include/linux/kernel.h

@@ -48,6 +48,8 @@
  */
 #define ARRAY_SIZE(arr) (sizeof(arr) / sizeof((arr)[0]) + __must_be_array(arr))
 
+#define PTR_IF(cond, ptr)	((cond) ? (ptr) : NULL)
+
 #define u64_to_user_ptr(x) (		\
 {					\
 	typecheck(u64, (x));		\