The Apple DART has a nifty feature that allows us protection of subranges

The Apple DART has a nifty feature that allows us protection of subranges
of a page with a granularity of 32-bit words.  Use this to expose just
those parts of memory to devices that we want the device to see.  This
means that handing down a small mbuf to a network card driver no longer
gives the hardware access to other mbufs in the same page.

It turns out that bge(4) always does aligned 64-bit access to memory though.
So round up/down to the nearest 64-bit boundary to prevent triggering an
IOMMU fault.

ok patrick@