s390/pci: Fix potential double remove of hotplug slot

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

s390/pci: Fix potential double remove of hotplug slot

In commit 6ee600bfbe0f ("s390/pci: remove hotplug slot when releasing the

device") the zpci_exit_slot() was moved from zpci_device_reserved() to

zpci_release_device() with the intention of keeping the hotplug slot

around until the device is actually removed.

Now zpci_release_device() is only called once all references are

dropped. Since the zPCI subsystem only drops its reference once the

device is in the reserved state it follows that zpci_release_device()

must only deal with devices in the reserved state. Despite that it

contains code to tear down from both configured and standby state. For

the standby case this already includes the removal of the hotplug slot

so would cause a double removal if a device was ever removed in

either configured or standby state.

Instead of causing a potential double removal in a case that should

never happen explicitly WARN_ON() if a device in non-reserved state is

released and get rid of the dead code cases.