Make bus_mem_add_mapping() return ENOMEM if we fail to allocate kva instead of panicing.
Make bus_mem_add_mapping() return ENOMEM if we fail to allocate kva instead
of panicing.

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