s390/pci: Support mmap() of PCI resources except for ISM devices

s390/pci: Support mmap() of PCI resources except for ISM devices

So far s390 does not allow mmap() of PCI resources to user-space via the
usual mechanisms, though it does use it for RDMA. For the PCI sysfs
resource files and /proc/bus/pci it defines neither HAVE_PCI_MMAP nor
ARCH_GENERIC_PCI_MMAP_RESOURCE. For vfio-pci s390 previously relied on
disabled VFIO_PCI_MMAP and now relies on setting pdev->non_mappable_bars
for all devices.

This is partly because access to mapped PCI resources from user-space
requires special PCI load/store memory-I/O (MIO) instructions, or the
special MMIO syscalls when these are not available. Still, such access is
possible and useful not just for RDMA, in fact not being able to mmap() PCI
resources has previously caused extra work when testing devices.

One thing that doesn't work with PCI resources mapped to user-space though
is the s390 specific virtual ISM device. Not only because the BAR size of
256 TiB prevents mapping the whole BAR but also because access requires use
of the legacy PCI instructions which are not accessible to user-space on
systems with the newer MIO PCI instructions.

Now with the pdev->non_mappable_bars flag ISM can be excluded from mapping
its resources while making this functionality available for all other PCI
devices. To this end introduce a minimal implementation of PCI_QUIRKS and
use that to set pdev->non_mappable_bars for ISM devices only. Then also set
ARCH_GENERIC_PCI_MMAP_RESOURCE to take advantage of the generic
implementation of pci_mmap_resource_range() enabling only the newer sysfs
mmap() interface. This follows the recommendation in
Documentation/PCI/sysfs-pci.rst.

Link: https://lore.kernel.org/r/20250226-vfio_pci_mmap-v7-3-c5c0f1d26efd@linux.ibm.com
Signed-off-by: Niklas Schnelle <schnelle@linux.ibm.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>