core/pci: Reserve PCI buses for RC's slot
core/pci: Reserve PCI buses for RC's slot

When RC's downstream link is down, we need reserve spare PCI buses
if it has an associated PCI hotplug slot. Otherwise, the adapter
behind it can't be probed successfully in PCI hot add scenario.

This reserves all available buses (to 255) for RC's hotplug slot
when its downstream is down so that PCI adapter can be hot added
to the slot afterwards.

Cc: stable # 5.3.0+
Signed-off-by: Gavin Shan <0398910e4f4970259b451f0f9d0686cc5e961b81@linux.vnet.ibm.com>
Signed-off-by: Stewart Smith <ec31ab75ddf977353c8f660f92ea8b23f64aef25@linux.vnet.ibm.com>

open-power/skiboot,open-power/skiboot,legoater/skiboot,shenki/skiboot,shenki/skiboot,qemu/skiboot,shenki/skiboot,open-power/skiboot,mikey/skiboot,shenki/skiboot,open-power/skiboot,qemu/skiboot,legoater/skiboot,legoater/skiboot,qemu/skiboot,stewart-ibm/skiboot,open-power/skiboot,qemu/skiboot,mikey/skiboot,qemu/skiboot,stewart-ibm/skiboot,legoater/skiboot,shenki/skiboot,stewart-ibm/skiboot,mikey/skiboot,legoater/skiboot