alpha: Silence -Warray-bounds warnings

GCC 11 (incorrectly[1]) assumes that literal values cast to (void *)
should be treated like a NULL pointer with an offset, and raises
diagnostics when doing bounds checking:

In function '__memset',
    inlined from '__bad_pagetable' at arch/alpha/mm/init.c:79:2:
./arch/alpha/include/asm/string.h:37:32: error: '__builtin_memset' offset [0, 8191] is out of the bounds [0, 0] [-Werror=array-bounds]
   37 |                         return __builtin_memset(s, c, n);
      |                                ^~~~~~~~~~~~~~~~~~~~~~~~~
In function '__memset',
    inlined from '__bad_page' at arch/alpha/mm/init.c:86:2:
./arch/alpha/include/asm/string.h:37:32: error: '__builtin_memset' offset [0, 8191] is out of the bounds [0, 0] [-Werror=array-bounds]
   37 |                         return __builtin_memset(s, c, n);
      |                                ^~~~~~~~~~~~~~~~~~~~~~~~~
In function '__memset',
    inlined from 'paging_init' at arch/alpha/mm/init.c:256:2:
./arch/alpha/include/asm/string.h:37:32: error: '__builtin_memset' offset [0, 8191] is out of the bounds [0, 0] [-Werror=array-bounds]
   37 |                         return __builtin_memset(s, c, n);

This has been solved in other places[2] already by using the recently
added absolute_pointer() macro. Do the same here.

[1] https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99578
[2] https://lore.kernel.org/all/20210912160149.2227137-1-linux@roeck-us.net/

Cc: Richard Henderson <rth@twiddle.net>
Cc: Ivan Kokshaysky <ink@jurassic.park.msu.ru>
Cc: Matt Turner <mattst88@gmail.com>
Cc: Guenter Roeck <linux@roeck-us.net>
Cc: linux-alpha@vger.kernel.org
Reviewed-and-tested-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Kees Cook <keescook@chromium.org>

Author: kees

arch/alpha/mm/init.c

@@ -76,14 +76,14 @@ pgd_alloc(struct mm_struct *mm)
 pmd_t *
 __bad_pagetable(void)
 {
-	memset((void *) EMPTY_PGT, 0, PAGE_SIZE);
+	memset(absolute_pointer(EMPTY_PGT), 0, PAGE_SIZE);
 	return (pmd_t *) EMPTY_PGT;
 }
 
 pte_t
 __bad_page(void)
 {
-	memset((void *) EMPTY_PGE, 0, PAGE_SIZE);
+	memset(absolute_pointer(EMPTY_PGE), 0, PAGE_SIZE);
 	return pte_mkdirty(mk_pte(virt_to_page(EMPTY_PGE), PAGE_SHARED));
 }
 
@@ -253,7 +253,7 @@ void __init paging_init(void)
 	free_area_init(max_zone_pfn);
 
 	/* Initialize the kernel's ZERO_PGE. */
-	memset((void *)ZERO_PGE, 0, PAGE_SIZE);
+	memset(absolute_pointer(ZERO_PGE), 0, PAGE_SIZE);
 }
 
 #if defined(CONFIG_ALPHA_GENERIC) || defined(CONFIG_ALPHA_SRM)