pci_pci: Support growing bus ranges in bus_adjust_resource for NEW_PCIB

This is the same underlying problem as 262459806433, just for bus ranges
rather than windows. SiFive's HiFive Unmatched has the following
topology:

Root Port <---> Bridge <---> Bridge <-+-> Bridge <---> (Unused)
 (pcib0)        (pcib1)      (pcib2)  |   (pcib3)
                                      +-> Bridge <---> xHCI
                                      |   (pcib4)
                                      +-> Bridge <---> M.2 E-key
                                      |   (pcib5)
                                      +-> Bridge <---> M.2 M-key
                                      |   (pcib6)
                                      +-> Bridge <---> x16 slot
                                          (pcib7)

If a device is plugged into the x16 slot that itself has a bridge, such
as many graphics cards, we currently fail to allocate a bus number for
its child bus (and so pcib_attach_child skips adding a child bus for
further enumeration) as, when the new child bridge attaches, it attempts
to allocate a bus number from its parent (pcib7) which in turn attempts
to grow its own bus range by calling bus_adjust_resource on its own
parent (pcib2) whose bus rman cannot accommodate the request and needs
to itself be extended by calling its own parent (pcib1). Note that
pcib3-7 do not face the same issue when they attach since pcib1 ends up
managing bus numbers 1-255 from the beginning and so never needs to grow
its own range.

Reviewed by: jhb, mav
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D32011