Background: pmap_object_init_pt() premaps the pages of a object in

Background: pmap_object_init_pt() premaps the pages of a object in
order to avoid the overhead of later page faults.  In general, it
implements two cases: one for vnode-backed objects and one for
device-backed objects.  Only the device-backed case is really
machine-dependent, belonging in the pmap.

This commit moves the vnode-backed case into the (relatively) new
function vm_map_pmap_enter().  On amd64 and i386, this commit only
amounts to code rearrangement.  On alpha and ia64, the new machine
independent (MI) implementation of the vnode case is smaller and more
efficient than their pmap-based implementations.  (The MI
implementation takes advantage of the fact that objects in -CURRENT
are ordered collections of pages.)  On sparc64, pmap_object_init_pt()
hadn't (yet) been implemented.