Clean up the code by converting xpmap_ptom(VTOP(...)) to VTOM(...) and
converting xpmap_ptom(VM_PAGE_TO_PHYS(...)) to VM_PAGE_TO_MACH(...). In
a few places we take advantage of the fact that xpmap_ptom can commute with
setting PG_* flags.
This commit should have no net effect save to improve the readability of
this code.
8cdabbaf32