gpio: virtuser: lock up configfs that an instantiated device depends on

gpio: virtuser: lock up configfs that an instantiated device depends on

Once a virtuser device is instantiated and actively used, allowing rmdir
for its configfs serves no purpose and can be confusing. Userspace
interacts with the virtual consumer at arbitrary times, meaning it
depends on its existence.

Make the subsystem itself depend on the configfs entry for a virtuser
device while it is in active use.

Signed-off-by: Koichiro Den <koichiro.den@canonical.com>
Link: https://lore.kernel.org/r/20250103141829.430662-4-koichiro.den@canonical.com
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>