gpiolib: fix bitmap operations related to line event watching

When operating on the bits of watched_lines bitmap, we're using
desc_to_gpio() which returns the GPIO number from the global numberspace.
This leads to all sorts of memory corruptions and invalid behavior. We
should switch to using gpio_chip_hwgpio() instead.

Fixes: 51c1064e ("gpiolib: add new ioctl() for monitoring changes in line info")
Reported-by: NKent Gibson <warthog618@gmail.com>
Signed-off-by: NBartosz Golaszewski <bgolaszewski@baylibre.com>
Tested-by: NKent Gibson <warthog618@gmail.com>

drivers/gpio/gpiolib.c

@@ -1261,7 +1261,7 @@ static long gpio_ioctl(struct file *filp, unsigned int cmd, unsigned long arg)
 			return -EFAULT;
 
 		if (cmd == GPIO_GET_LINEINFO_WATCH_IOCTL)
-			set_bit(desc_to_gpio(desc), priv->watched_lines);
+			set_bit(gpio_chip_hwgpio(desc), priv->watched_lines);
 
 		return 0;
 	} else if (cmd == GPIO_GET_LINEHANDLE_IOCTL) {
@@ -1276,7 +1276,7 @@ static long gpio_ioctl(struct file *filp, unsigned int cmd, unsigned long arg)
 		if (IS_ERR(desc))
 			return PTR_ERR(desc);
 
-		clear_bit(desc_to_gpio(desc), priv->watched_lines);
+		clear_bit(gpio_chip_hwgpio(desc), priv->watched_lines);
 		return 0;
 	}
 	return -EINVAL;
@@ -1304,7 +1304,7 @@ static int lineinfo_changed_notify(struct notifier_block *nb,
 	struct gpio_desc *desc = data;
 	int ret;
 
-	if (!test_bit(desc_to_gpio(desc), priv->watched_lines))
+	if (!test_bit(gpio_chip_hwgpio(desc), priv->watched_lines))
 		return NOTIFY_DONE;
 
 	memset(&chg, 0, sizeof(chg));