Allow pmap_early_io_unmap() to reclaim memory

pmap_early_io_map()/pmap_early_io_unmap(), if used in pairs, should be used in
the form:

pmap_early_io_map()
..do stuff..
pmap_early_io_unmap()

Without other allocations in the middle. Without reclaiming memory this can
leave large holes in the device space.

While here, make a simple change to the unmap loop which now permits it to unmap
multiple TLB entries in the range.