and out of kernel address space (via the pass(4) and xpt(4) peripheral
drivers) to 64K (DFLTPHYS). Some controllers, like the Adaptec 1542,
don't support more than 64K transactions.
We plan on eventually having the capability of limiting this size based
on min(MAXPHYS, controller max), but since that capability isn't here yet,
limit things to the lowest common denominator.