in _bus_dmamem_map use PMAP_CANFAIL for mapping in the pages.
in _bus_dmamem_map use PMAP_CANFAIL for mapping in the pages.

bus_dmamem_map can fail for a myriad of reasons already, so panicing if
pmap fails is just impolite. other archs will be forthcoming.

ok krw@, art@ before lock

orumin/openbsd-efivars,orumin/openbsd-efivars,orumin/openbsd-efivars,orumin/openbsd-efivars