kunit/memcpy: Avoid pathological compile-time string size

The memcpy() KUnit tests are trying to sanity-check run-time behaviors,
but tripped compile-time warnings about a pathological condition of a
too-small buffer being used for input. Avoid this by explicitly resizing
the buffer, but leaving the string short. Avoid the following warning:

lib/memcpy_kunit.c: In function 'strtomem_test':
include/linux/string.h:303:42: warning: 'strnlen' specified bound 4 exceeds source size 3 [-Wstringop-overread]
303 | memcpy(dest, src, min(_dest_len, strnlen(src, _dest_len))); \
include/linux/minmax.h:32:39: note: in definition of macro '__cmp_once'
32 | typeof(y) unique_y = (y); \
| ^
include/linux/minmax.h:45:25: note: in expansion of macro '__careful_cmp'
45 | #define min(x, y) __careful_cmp(x, y, <)
| ^~~~~~~~~~~~~
include/linux/string.h:303:27: note: in expansion of macro 'min'
303 | memcpy(dest, src, min(_dest_len, strnlen(src, _dest_len))); \
| ^~~
lib/memcpy_kunit.c:290:9: note: in expansion of macro 'strtomem'
290 | strtomem(wrap.output, input);
| ^~~~~~~~
lib/memcpy_kunit.c:275:27: note: source object allocated here
275 | static const char input[] = "hi";
| ^~~~~

Reported-by: kernel test robot <[e-mail]>
Link: https://lore.kernel.org/linux-mm/[e-mail]
Fixes: dfbafa70bde2 ("string: Introduce strtomem() and strtomem_pad()")
Signed-off-by: Kees Cook <[e-mail]>

lib/memcpy_kunit.c

···
 
 static void strtomem_test(struct kunit *test)
 {
-	static const char input[] = "hi";
+	static const char input[sizeof(unsigned long)] = "hi";
 	static const char truncate[] = "this is too long";
 	struct {
 		unsigned long canary1;