Properly handle alignment requests bigger that page size.

• for now, alignments bigger that page size is allowed only for buffers allocated by bus_dmamem_alloc(), cover this fact by KASSERT.
• never bounce buffers allocated by bus_dmamem_alloc(), these always comply with the required rules (alignment, boundary, address range).

MFC after: 1 week
Reviewed by: jah
PR: 235542