MFC r232267:

MFC r232267:
  Workaround for PCIe 4GB boundary issue

  Enforce a boundary of no more than 4GB - transfers crossing a 4GB
  boundary can lead to data corruption due to PCIe limitations.  This
  change is a less-intrusive workaround that can be quickly merged back
  to older branches; a cleaner implementation will arrive in HEAD later
  but may require KPI changes.

  This change is based on a suggestion by jhb@.

Sponsored by:   Sandvine Incorporated

git-svn-id: svn://svn.freebsd.org/base/stable/9@232372 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f