PM / QoS: Improve sysfs pm_qos_latency_tolerance validation

Negative values are special. Don't let users write them directly.

Signed-off-by: Andy Lutomirski <[e-mail]>
Signed-off-by: Rafael J. Wysocki <[e-mail]>

drivers/base/power/sysfs.c

···
 	s32 value;
 	int ret;
 
-	if (kstrtos32(buf, 0, &value)) {
+	if (kstrtos32(buf, 0, &value) == 0) {
+		/* Users can't write negative values directly */
+		if (value < 0)
+			return -EINVAL;
+	} else {
 		if (!strcmp(buf, "auto") || !strcmp(buf, "auto\n"))
 			value = PM_QOS_LATENCY_TOLERANCE_NO_CONSTRAINT;
 		else if (!strcmp(buf, "any") || !strcmp(buf, "any\n"))