CVE-2026-89795: PCI: Allow per function PCI slots to fix slot reset on s390

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

PCI: Allow per function PCI slots to fix slot reset on s390

On s390 systems, which use a machine level hypervisor, PCI devices are always accessed through a form of PCI pass-through which fundamentally operates on a per PCI function granularity. This is also reflected in the s390 PCI hotplug driver which creates hotplug slots for individual PCI functions. Its resetslot() function, which is a wrapper for zpcihotresetdevice(), thus also resets individual functions.

Currently, the pcicreateslot() assigns the same pcislot object to multifunction devices. This approach worked fine on s390 systems that only exposed virtual functions as individual PCI domains to the operating system. Since commit 44510d6fa0c0 ("s390/pci: Handling multifunctions") s390 supports exposing the topology of multifunction PCI devices by grouping them in a shared PCI domain. This creates a problem when resetting a function through the hotplug driver's slotreset() interface.

When attempting to reset a function through the hotplug driver, the shared slot assignment causes the wrong function to be reset instead of the intended one. It also leaks memory as we do create a pcislot object for the function, but don't correctly free it in pcislotrelease().

Add a flag for struct pcislot to allow per function PCI slots for functions managed through a hypervisor, which exposes individual PCI functions while retaining the topology. Since we can use all 8 bits for slot 'number' (for ARI devices), change slot 'number' u16 to account for special values PCISLOTPLACEHOLDER and PCISLOTALLDEVICES.