vm_page: Reorder some fields

[markj]

Summary

With listq removed, UMA can start using the first 32 bytes of struct
vm_page to store a slab header. Reorder some fields to make this
possible. In particular, move pindex to offset 0 so that the pctrie
implementation doesn't need to add an offset to vm_page pointers.

It's ok for a noobj page consumer to use the ref_count and busy_lock
fields for their own purposes, so move those too.