stackinit: Keep selftest union size small on m68k

The stack frame on m68k is very sensitive to the size of what needs to
be stored. Like done for long string testing, reduce the size of the
large trailing struct in the union initialization testing.

Reported-by: Geert Uytterhoeven <geert@linux-m68k.org>
Closes: https://lore.kernel.org/all/CAMuHMdXW8VbtOAixO7w+aDOG70aZtZ50j1Ybcr8B3eYnRUcrcA@mail.gmail.com
Fixes: e71a29d ("stackinit: Add union initialization to selftests")
Link: https://lore.kernel.org/r/20250204174509.work.711-kees@kernel.org
Signed-off-by: Kees Cook <kees@kernel.org>
Tested-by: Geert Uytterhoeven <geert@linux-m68k.org>

lib/stackinit_kunit.c

@@ -75,8 +75,10 @@ static bool stackinit_range_contains(char *haystack_start, size_t haystack_size,
  */
 #ifdef CONFIG_M68K
 #define FILL_SIZE_STRING		8
+#define FILL_SIZE_ARRAY			2
 #else
 #define FILL_SIZE_STRING		16
+#define FILL_SIZE_ARRAY			8
 #endif
 
 #define INIT_CLONE_SCALAR		/**/
@@ -345,7 +347,7 @@ union test_small_start {
 	short three;
 	unsigned long four;
 	struct big_struct {
-		unsigned long array[8];
+		unsigned long array[FILL_SIZE_ARRAY];
 	} big;
 };