PCI: Release resource invalidated by coalescing

When contiguous windows are coalesced by pci_register_host_bridge(), the
second resource is expanded to include the first, and the first is
invalidated and consequently not added to the bus. However, it remains in
the resource hierarchy.  For example, these windows:

  fec00000-fec7ffff : PCI Bus 0000:00
  fec80000-fecbffff : PCI Bus 0000:00

are coalesced into this, where the first resource remains in the tree with
start/end zeroed out:

  00000000-00000000 : PCI Bus 0000:00
  fec00000-fecbffff : PCI Bus 0000:00

In some cases (e.g. the Xen scratch region), this causes future calls to
allocate_resource() to choose an inappropriate location which the caller
cannot handle.

Fix by releasing the zeroed-out resource and removing it from the resource
hierarchy.

[bhelgaas: commit log]
Fixes: 7c3855c ("PCI: Coalesce host bridge contiguous apertures")
Link: https://lore.kernel.org/r/20230525153248.712779-1-ross.lagerwall@citrix.com
Signed-off-by: Ross Lagerwall <ross.lagerwall@citrix.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Cc: stable@vger.kernel.org	# v5.16+

drivers/pci/probe.c

@@ -997,8 +997,10 @@ static int pci_register_host_bridge(struct pci_host_bridge *bridge)
 	resource_list_for_each_entry_safe(window, n, &resources) {
 		offset = window->offset;
 		res = window->res;
-		if (!res->flags && !res->start && !res->end)
+		if (!res->flags && !res->start && !res->end) {
+			release_resource(res);
 			continue;
+		}
 
 		list_move_tail(&window->node, &bridge->windows);