PCI: Equalize hotplug memory and io for occupied and empty slots

Currently, a hotplug bridge will be given hpmemsize additional memory
and hpiosize additional io if available, in order to satisfy any future
hotplug allocation requirements.

These calculations don't consider the current memory/io size of the
hotplug bridge/slot, so hotplug bridges/slots which have downstream
devices will be allocated their current allocation in addition to the
hpmemsize value.

This makes for possibly undesirable results with a mix of unoccupied and
occupied slots (ex, with hpmemsize=2M):

  02:03.0 PCI bridge: <-- Occupied
	  Memory behind bridge: d6200000-d64fffff [size=3M]
  02:04.0 PCI bridge: <-- Unoccupied
	  Memory behind bridge: d6500000-d66fffff [size=2M]

This change considers the current allocation size when using the
hpmemsize/hpiosize parameters to make the reservations predictable for
the mix of unoccupied and occupied slots:

  02:03.0 PCI bridge: <-- Occupied
	  Memory behind bridge: d6200000-d63fffff [size=2M]
  02:04.0 PCI bridge: <-- Unoccupied
	  Memory behind bridge: d6400000-d65fffff [size=2M]

Signed-off-by: Jon Derrick <jonathan.derrick@intel.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>