While the GPIO cdev gpio_ioctl() call is in progress, the kernel can
call gpiochip_remove() which will set gdev->chip to NULL, after which
any subsequent access will cause a crash.

gpio_ioctl() was overlooked by the previous fix to protect syscalls
(bdbbae24), so add protection for that.

Fixes: bdbbae24 ("gpiolib: protect the GPIO device against being dropped while in use by user-space")
Fixes: d7c51b47 ("gpio: userspace ABI for reading/writing GPIO lines")
Fixes: 3c0d9c63 ("gpiolib: cdev: support GPIO_V2_GET_LINE_IOCTL and GPIO_V2_LINE_GET_VALUES_IOCTL")
Fixes: aad95584 ("gpiolib: cdev: support GPIO_V2_GET_LINEINFO_IOCTL and GPIO_V2_GET_LINEINFO_WATCH_IOCTL")
Signed-off-by: Kent Gibson <warthog618@gmail.com>
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>