At Justin's request, limit the size of buffers that can be mapped into

At Justin's request, limit the size of buffers that can be mapped into
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.