Don't destroy bus_dma maps created by bus_dmamem_alloc().  In some cases,
don't create a map before calling bus_dmamem_alloc() (such maps were
leaked).  It is believed that the extra destroy of the map was generally
harmless since bus_dmamem_alloc() often uses special maps for which
bus_dmamap_destroy() is a no-op (e.g. on x86).

Reviewed by:	scottl