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
3f8e262e8c