debugfs.c: extend example with toplevel file, fop and kstrtoull_from_user

kernel_module/hello.c

@@ -14,6 +14,9 @@ Hello world module.
 static int myinit(void)
 {
 	pr_info("hello init\n");
+	/* 0 for success, any negative value means failure,
+	 * E* consts if you want to specify failure cause.
+	 * https://www.linux.com/learn/kernel-newbie-corner-loadable-kernel-modules-coming-and-going */
 	return 0;
 }