MFC 237008,237271,237272,237673:

MFC 237008,237271,237272,237673:
- Fix a couple of bugs that prevented windows in PCI-PCI bridges from
  growing "downward" (moving the start address down).  First, an off by
  one error caused the end address to be moved down an extra alignment
  chunk unnecessarily.  Second, when aligning the new candidate starting
  address, the wrong bits were masked off.
- Add a 'wmask' variable to hold the expression '(1ul << w->step) - 1' in
  pcib_grow_window().
- For subtractively decoding bridges, don't try to grow windows but pass
  the request up the tree in order to be on the safe side. Growing windows
  in this case would mean to switch resources to positive decoding and
  it's unclear how to correctly handle this. At least with ALi/ULi M5249
  PCI-PCI bridges, this also just doesn't work out of the box.

git-svn-id: svn://svn.freebsd.org/base/stable/9@237797 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f