gpiolib: fix memory leak in gpiochip_setup_dev()

Summary

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.