kasan/test: fix protection against compiler elision

The kunit test is using assignments to
"static volatile void *kasan_ptr_result" to prevent elision of memory
loads, but that's not working:
In this variable definition, the "volatile" applies to the "void", not to
the pointer.
To make "volatile" apply to the pointer as intended, it must follow
after the "*".

This makes the kasan_memchr test pass again on my system.  The
kasan_strings test is still failing because all the definitions of
load_unaligned_zeropad() are lacking explicit instrumentation hooks and
ASAN does not instrument asm() memory operands.

Link: https://lkml.kernel.org/r/20250728-kasan-kunit-fix-volatile-v1-1-e7157c9af82d@google.com
Fixes: 5f1c810 ("mm:kasan: fix sparse warnings: Should it be static?")
Signed-off-by: Jann Horn <jannh@google.com>
Cc: Alexander Potapenko <glider@google.com>
Cc: Andrey Konovalov <andreyknvl@gmail.com>
Cc: Andrey Ryabinin <ryabinin.a.a@gmail.com>
Cc: Dmitriy Vyukov <dvyukov@google.com>
Cc: Jann Horn <jannh@google.com>
Cc: Nihar Chaithanya <niharchaithanya@gmail.com>
Cc: Vincenzo Frascino <vincenzo.frascino@arm.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

Author: thejh

mm/kasan/kasan_test_c.c

@@ -47,7 +47,7 @@ static struct {
  * Some tests use these global variables to store return values from function
  * calls that could otherwise be eliminated by the compiler as dead code.
  */
-static volatile void *kasan_ptr_result;
+static void *volatile kasan_ptr_result;
 static volatile int kasan_int_result;
 
 /* Probe for console output: obtains test_status lines of interest. */