startup_alloc() uses pmap_map() to map slabs used for early allocations.
pmap_map() may ignore the hint address and simply return a range from
the direct map. In this case we must not unmap the range in
UMA uses bootstart and bootmem to track the range of KVA into which
slabs are mapped if the direct map is not used. Unmap a startup slab
only if it was mapped into that range.