MFC: r206448

MFC: r206448

Do as the comment suggests and determine the bus space based on the last
bus we actually mapped at rather than always based on the last bus we
encountered while moving upward in the tree. Otherwise we might use the
wrong bus space in case the bridge directly underneath the nexus doesn't
require mapping, i.e. was skipped as it's the case for ssm(4) nodes.

git-svn-id: svn://svn.freebsd.org/base/stable/8@207518 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f