uapi/linux/const.h: prefer ISO-friendly __typeof__

typeof is (still) a GNU extension, which means that it cannot be used when
building ISO C (e.g. -std=c99). It should therefore be avoided in uapi
headers in favour of the ISO-friendly __typeof__.

Unfortunately this issue could not be detected by
CONFIG_UAPI_HEADER_TEST=y as the __ALIGN_KERNEL() macro is not expanded in
any uapi header.

This matters from a userspace perspective, not a kernel one. uapi
headers and their contents are expected to be usable in a variety of
situations, and in particular when building ISO C applications (with
-std=c99 or similar).

This particular problem can be reproduced by trying to use the
__ALIGN_KERNEL macro directly in application code, say:

#include <linux/const.h>

int align(int x, int a)
{
return __KERNEL_ALIGN(x, a);
}

and trying to build that with -std=c99.

Link: https://lkml.kernel.org/r/[e-mail]
Fixes: a79ff731a1b2 ("netfilter: xtables: make XT_ALIGN() usable in exported headers by exporting __ALIGN_KERNEL()")
Signed-off-by: Kevin Brodsky <[e-mail]>
Reported-by: Ruben Ayrapetyan <[e-mail]>
Tested-by: Ruben Ayrapetyan <[e-mail]>
Reviewed-by: Petr Vorel <[e-mail]>
Tested-by: Petr Vorel <[e-mail]>
Reviewed-by: Masahiro Yamada <[e-mail]>
Cc: Sam Ravnborg <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>

include/uapi/linux/const.h

···
 #define _BITUL(x)	(_UL(1) << (x))
 #define _BITULL(x)	(_ULL(1) << (x))
 
-#define __ALIGN_KERNEL(x, a)		__ALIGN_KERNEL_MASK(x, (typeof(x))(a) - 1)
+#define __ALIGN_KERNEL(x, a)		__ALIGN_KERNEL_MASK(x, (__typeof__(x))(a) - 1)
 #define __ALIGN_KERNEL_MASK(x, mask)	(((x) + (mask)) & ~(mask))
 
 #define __KERNEL_DIV_ROUND_UP(n, d) (((n) + (d) - 1) / (d))