gpio: sysfs: fix chip removal with GPIOs exported over sysfs

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

gpio: sysfs: fix chip removal with GPIOs exported over sysfs

Currently if we export a GPIO over sysfs and unbind the parent GPIO

controller, the exported attribute will remain under /sys/class/gpio

because once we remove the parent device, we can no longer associate the

descriptor with it in gpiod_unexport() and never drop the final

reference.

Rework the teardown code: provide an unlocked variant of

gpiod_unexport() and remove all exported GPIOs with the sysfs_lock taken

before unregistering the parent device itself. This is done to prevent

any new exports happening before we unregister the device completely.