kunit: Add printf attribute to fail_current_test_impl

Add the gnu_printf (__printf()) attribute to the
kunit_fail_current_test() implementation in
__kunit_fail_current_test_impl(). While it's not actually useful here,
as this function is never called directly, it nevertheless was
triggering -Wsuggest-attribute=format warnings, so we should add it to
reduce the noise.

Fixes: cc3ed2fe5c93 ("kunit: Add "hooks" to call into KUnit when it's built as a module")
Reported-by: kernel test robot <[e-mail]>
Signed-off-by: David Gow <[e-mail]>
Reviewed-by: Rae Moar <[e-mail]>
Signed-off-by: Shuah Khan <[e-mail]>

lib/kunit/hooks-impl.h

···
 #include <kunit/test-bug.h>
 
 /* List of declarations. */
-void __kunit_fail_current_test_impl(const char *file, int line, const char *fmt, ...);
+void __printf(3, 4) __kunit_fail_current_test_impl(const char *file,
+						   int line,
+						   const char *fmt, ...);
 void *__kunit_get_static_stub_address_impl(struct kunit *test, void *real_fn_addr);
 
 /* Code to set all of the function pointers. */

lib/kunit/test.c

···
 /*
  * Hook to fail the current test and print an error message to the log.
  */
-void __kunit_fail_current_test_impl(const char *file, int line, const char *fmt, ...)
+void __printf(3, 4) __kunit_fail_current_test_impl(const char *file, int line, const char *fmt, ...)
 {
 	va_list args;
 	int len;