MFC r276867:

MFC r276867:
Fix DMAR context allocations for the devices behind PCIe->PCI bridges
after dmar driver was converted to use rids.  The bus component to
calculate context page must be taken from the requestor rid, which is
a bridge, and not from the device bus number.

git-svn-id: svn://svn.freebsd.org/base/stable/10@279484 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f