PCI: Add pci_bus_clip_resource() to clip to fit upstream window

Add pci_bus_clip_resource().  If a PCI-PCI bridge window overlaps an
upstream bridge window but is not completely contained by it, this clips
the downstream window so it fits inside the upstream one.

No functional change (this adds the function but no callers).

[bhelgaas: changelog, split into separate patch]
Link: https://bugzilla.kernel.org/show_bug.cgi?id=85491
Reported-by: Marek Kordik <kordikmarek@gmail.com>
Fixes: 5b28541552 ("PCI: Restrict 64-bit prefetchable bridge windows to 64-bit resources")
Signed-off-by: Yinghai Lu <yinghai@kernel.org>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
CC: stable@vger.kernel.org	# v3.16+

drivers/pci/pci.h

@@ -208,6 +208,7 @@ void __pci_bus_size_bridges(struct pci_bus *bus,
 void __pci_bus_assign_resources(const struct pci_bus *bus,
 				struct list_head *realloc_head,
 				struct list_head *fail_head);
+bool pci_bus_clip_resource(struct pci_dev *dev, int idx);
 
 /**
  * pci_ari_enabled - query ARI forwarding status