gpiolib: fix memory leak in gpiochip_setup_dev()

In the Linux kernel, the following vulnerability has been resolved:

gpiolib: fix memory leak in gpiochip_setup_dev()

Here is a backtrace report about memory leak detected in

gpiochip_setup_dev():

unreferenced object 0xffff88810b406400 (size 512):

comm "python3", pid 1682, jiffies 4295346908 (age 24.090s)

backtrace:

kmalloc_trace

device_add device_private_init at drivers/base/core.c:3361

(inlined by) device_add at drivers/base/core.c:3411

cdev_device_add

gpiolib_cdev_register

gpiochip_setup_dev

gpiochip_add_data_with_key

gcdev_register() & gcdev_unregister() would call device_add() &

device_del() (no matter CONFIG_GPIO_CDEV is enabled or not) to

register/unregister device.

However, if device_add() succeeds, some resource (like

struct device_private allocated by device_private_init())

is not released by device_del().

Therefore, after device_add() succeeds by gcdev_register(), it

needs to call put_device() to release resource in the error handle

path.

Here we move forward the register of release function, and let it

release every piece of resource by put_device() instead of kfree().

While at it, fix another subtle issue, i.e. when gc->ngpio is equal

to 0, we still call kcalloc() and, in case of further error, kfree()

on the ZERO_PTR pointer, which is not NULL. It's not a bug per se,

but rather waste of the resources and potentially wrong expectation

about contents of the gdev->descs variable.