Make vm_page_array setting dynamic in the PMAP_HAS_PAGE_ARRAY case

[jhibbits]

Summary

Some platforms may want to dynamically allocate the mapping address for
vm_page_array, while controlling the mapping.

[kib]

This is fine in principle but I do not see why cannot you reset the value of vm_page_array.

Also, I would suggest use some macro for vm_page_array initial value and conditionalize the initialization on that macro. Or even, all arches might define it, to zero unless some other value is needed.

[jhibbits]

You're right, I don't know what I was thinking. The page_range change is the only thing needed from this diff.