fortify: Use C arithmetic not FIELD_xxx() in FORTIFY_REASON defines

FIELD_GET() and FIELD_PREP() are mainly useful for hardware register
accesses, but here they are being used for some very simple oprations.

This wouldn't matter much, but they contain a lot of compile-time
checks (that really aren't needed here) that bloat the expansion
of FIELD_GET(GENMASK(7, 1), func) to over 18KB.
Even with the 'bloat reduced' FIELD_GET/PREP they are still hundreds of
characters.

Replace FIELD_GET(BIT(0), r) with ((r) & 1), FIELD_GET(GENMASK(7, 1), r) with
(r) >> 1), and (FIELD_PREP(BIT(0), write) | FIELD_PREP(GENMASK(7, 1), func))
with ((func) << 1 | (write)).

The generated code is the same, but it makes the .c file less obfuctaced,
the .i file much easier to read, and should marginally decrease compilation
time.

Signed-off-by: David Laight <david.laight.linux@gmail.com>
Link: https://patch.msgid.link/20251214125857.3308-1-david.laight.linux@gmail.com
Signed-off-by: Kees Cook <kees@kernel.org>

Author: David Laight

include/linux/fortify-string.h

@@ -2,18 +2,16 @@
 #ifndef _LINUX_FORTIFY_STRING_H_
 #define _LINUX_FORTIFY_STRING_H_
 
-#include <linux/bitfield.h>
 #include <linux/bug.h>
 #include <linux/const.h>
 #include <linux/limits.h>
 
 #define __FORTIFY_INLINE extern __always_inline __gnu_inline __overloadable
 #define __RENAME(x) __asm__(#x)
 
-#define FORTIFY_REASON_DIR(r)		FIELD_GET(BIT(0), r)
-#define FORTIFY_REASON_FUNC(r)		FIELD_GET(GENMASK(7, 1), r)
-#define FORTIFY_REASON(func, write)	(FIELD_PREP(BIT(0), write) | \
-					 FIELD_PREP(GENMASK(7, 1), func))
+#define FORTIFY_REASON_DIR(r)		((r) & 1)
+#define FORTIFY_REASON_FUNC(r)		((r) >> 1)
+#define FORTIFY_REASON(func, write)	((func) << 1 | (write))
 
 /* Overridden by KUnit tests. */
 #ifndef fortify_panic