s390/vdso: reuse kstrtobool for option value parsing

"vdso" option setup already recognises integer and textual values. Yet
kstrtobool is a more common way to parse boolean values, reuse it to
unify option value parsing behavior and simplify code a bit.

While at it, __setup value parsing callbacks are expected to return
1 when an option is recognized, and returning any other value won't
trigger any error message currently, so simply return 1.

Also don't change default vdso_enabled value of 1 when "vdso" option
value is invalid.
Reviewed-by: Philipp Rudo <prudo@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>

arch/s390/kernel/vdso.c

@@ -97,21 +97,13 @@ static const struct vm_special_mapping vdso_mapping = {
 	.mremap = vdso_mremap,
 };
 
-static int __init vdso_setup(char *s)
+static int __init vdso_setup(char *str)
 {
-	unsigned long val;
-	int rc;
+	bool enabled;
 
-	rc = 0;
-	if (strncmp(s, "on", 3) == 0)
-		vdso_enabled = 1;
-	else if (strncmp(s, "off", 4) == 0)
-		vdso_enabled = 0;
-	else {
-		rc = kstrtoul(s, 0, &val);
-		vdso_enabled = rc ? 0 : !!val;
-	}
-	return !rc;
+	if (!kstrtobool(str, &enabled))
+		vdso_enabled = enabled;
+	return 1;
 }
 __setup("vdso=", vdso_setup);