s390/pci: fix use after free of zpci_dev

s390/pci: fix use after free of zpci_dev

The struct pci_dev uses reference counting but zPCI assumed erroneously
that the last reference would always be the local reference after
calling pci_stop_and_remove_bus_device(). This is usually the case but
not how reference counting works and thus inherently fragile.

In fact one case where this causes a NULL pointer dereference when on an
SRIOV device the function 0 was hot unplugged before another function of
the same multi-function device. In this case the second function's
pdev->sriov->dev reference keeps the struct pci_dev of function 0 alive
even after the unplug. This bug was previously hidden by the fact that
we were leaking the struct pci_dev which in turn means that it always
outlived the struct zpci_dev. This was fixed in commit 0b13525c20fe
("s390/pci: fix leak of PCI device structure") exposing the broken
behavior.

Fix this by accounting for the long living reference a struct pci_dev
has to its underlying struct zpci_dev via the zbus->function[] array and
only release that in pcibios_release_device() ensuring that the struct
pci_dev is not left with a dangling reference. This is a minimal fix in
the future it would probably better to use fine grained reference
counting for struct zpci_dev.

Fixes: 05bc1be6db4b2 ("s390/pci: create zPCI bus")
Cc: stable@vger.kernel.org
Reviewed-by: Matthew Rosato <mjrosato@linux.ibm.com>
Signed-off-by: Niklas Schnelle <schnelle@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>