gpio: virtuser: fix UAF in configfs release path

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

gpio: virtuser: fix UAF in configfs release path

The gpio-virtuser configfs release path uses guard(mutex) to protect the device structure. However, the device is freed before the guard cleanup runs, causing mutex_unlock() to operate on freed memory.

Specifically, gpio_virtuser_device_config_group_release() destroys the mutex and frees the device while still inside the guard(mutex) scope. When the function returns, the guard cleanup invokes mutex_unlock(&dev->lock), resulting in a slab use-after-free.

Limit the mutex lifetime by using a scoped_guard() only around the activation check, so that the lock is released before mutex_destroy() and kfree() are called.