s390/pci: Fix leak of struct zpci_dev when zpci_add_device() fails

Prior to commit 0467cdde ("s390/pci: Sort PCI functions prior to
creating virtual busses") the IOMMU was initialized and the device was
registered as part of zpci_create_device() with the struct zpci_dev
freed if either resulted in an error. With that commit this was moved
into a separate function called zpci_add_device().

While this new function logs when adding failed, it expects the caller
not to use and to free the struct zpci_dev on error. This difference
between it and zpci_create_device() was missed while changing the
callers and the incompletely initialized struct zpci_dev may get used in
zpci_scan_configured_device in the error path. This then leads to
a crash due to the device not being registered with the zbus. It was
also not freed in this case. Fix this by handling the error return of
zpci_add_device(). Since in this case the zdev was not added to the
zpci_list it can simply be discarded and freed. Also make this more
explicit by moving the kref_init() into zpci_add_device() and document
that zpci_zdev_get()/zpci_zdev_put() must be used after adding.

Cc: stable@vger.kernel.org
Fixes: 0467cdde ("s390/pci: Sort PCI functions prior to creating virtual busses")
Reviewed-by: Gerd Bayer <gbayer@linux.ibm.com>
Reviewed-by: Matthew Rosato <mjrosato@linux.ibm.com>
Signed-off-by: Niklas Schnelle <schnelle@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>