Workaround for PCIe 4GB boundary issue

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@.

Reviewed by:    scottl, jhb
Sponsored by:   Sandvine Incorporated
MFC after:      3 days