gpio: virtuser: fix UAF in configfs release path

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.

Fixes: 91581c4b3f29 ("gpio: virtuser: new virtual testing driver for the GPIO API")
Signed-off-by: Yuhao Huang <nekowong743@gmail.com>
Link: https://lore.kernel.org/r/20260126040348.11167-1-yuhaohuang@YuhaodeMacBook-Pro.local
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@oss.qualcomm.com>