Fix loading memory allocated with bus_dmamem_alloc(9). The old could would

Fix loading memory allocated with bus_dmamem_alloc(9).  The old could would
always load all allocated pages instead of the size specified in the
bus_dmamap_load_raw(9) call.  Also fixes the corner case where a specified
boundary is less than the page size, which would always create multiple
segments, even if the specified size was smaller than the boundary.

Fixes xhci(4) on sparc64.