Partial workaround for Intel PCI adapters reading past the end of the
host-programmed DMA regions. This change seemingly fixes the
descriptor fetches, but the packet memory accesses are left
problematic.

Reviewed by: emaste, erj, sbruno
Sponsored by: The FreeBSD Foundation
MFC after: 2 weeks
Differential revision: https://reviews.freebsd.org/D8282