gpio: use kzalloc to allocate gpio_device

The use of kmalloc() to allocate the gpio_device leaves the contained struct
device object in an unknown state.  Calling dev_set_name() on a struct device
of unknown state can trigger the free() of an invalid pointer, as seen in the
following backtrace (collected by Tony Lindgren):

  kfree
  kobject_set_name_vargs
  dev_set_name
  gpiochip_add_data
  omap_gpio_probe
  platform_drv_probe
  ...
Reported-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reported-by: Michael Welling <mwelling@ieee.org>
Reported-by: Tony Lindgren <tony@atomide.com>
Tested-by: Michael Welling <mwelling@ieee.org>
Tested-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Josh Cartwright <joshc@ni.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>

drivers/gpio/gpiolib.c

@@ -435,7 +435,7 @@ int gpiochip_add_data(struct gpio_chip *chip, void *data)
 	 * First: allocate and populate the internal stat container, and
 	 * set up the struct device.
 	 */
-	gdev = kmalloc(sizeof(*gdev), GFP_KERNEL);
+	gdev = kzalloc(sizeof(*gdev), GFP_KERNEL);
 	if (!gdev)
 		return -ENOMEM;
 	gdev->dev.bus = &gpio_bus_type;