PCI: Don't scan random busses in pci_scan_bridge()

When assigning a new bus number in pci_scan_bridge we check whether
max+1 is free by calling pci_find_bus. If it does already exist then we
assume that we are rescanning and that this is the right bus to scan.

This is fragile. If max+1 lies outside of bus->busn_res.end then we will
rescan some random bus from somewhere else in the hierachy. This patch
checks for this case and prints a warning.

[bhelgaas: add parent/child bus number info to dev_warn()]
Signed-off-by: Andreas Noever <andreas.noever@gmail.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>