CVE-2024-56699: 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 zpciexitslot() was moved from zpcidevicereserved() to zpcireleasedevice() with the intention of keeping the hotplug slot around until the device is actually removed.

Now zpcireleasedevice() 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 zpcireleasedevice() 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 WARNON() if a device in non-reserved state is released and get rid of the dead code cases.