Changeset 136

pmap_extract(pmap_kernel()) can be called in interrupt context via
bus_dma functions, so we can't take a sleep lock in this case.
As the kernel shadow PD is always present in all pmaps we don't
need to explicitely map pmap_kernel(), and we don't need to lock pmaps
either (if the mapped pmap is going to be destroyed, just remplace
it with pmap_kernel() in the mapping pmap).