s390/pci: expose a PCI device's UID as its index

On s390 each PCI device has a user-defined ID (UID) exposed under
/sys/bus/pci/devices/<dev>/uid. This ID was designed to serve as the PCI
device's primary index and to match the device within Linux to the
device configured in the hypervisor. To serve as a primary identifier
the UID must be unique within the Linux instance, this is guaranteed by
the platform if and only if the UID Uniqueness Checking flag is set
within the CLP List PCI Functions response.

In this sense the UID serves an analogous function as the SMBIOS
instance number or ACPI index exposed as the "index" respectively
"acpi_index" device attributes and used by e.g. systemd to set interface
names. As s390 does not use and will likely never use ACPI nor SMBIOS
there is no conflict and we can just expose the UID under the "index"
attribute whenever UID Uniqueness Checking is active and get systemd's
interface naming support for free.

Link: https://lore.kernel.org/lkml/20210412135905.1434249-1-schnelle@linux.ibm.com/
Acked-by: Viktor Mihajlovski <mihajlov@linux.ibm.com>
Acked-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Narendra K <narendra_k@dell.com>
Signed-off-by: Niklas Schnelle <schnelle@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>

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