struct vmspace is freed from cpu_sched_exit() to pmap_release().
This has the advantage of being able to rely on MI code to decide
when a free should occur, instead of having to inspect the reference
count ourselves.
At the same time, turn the per-CPU vmspace pointer into a pmap pointer,
so that pmap_release() can deal with pmaps exclusively.
Reviewed (and embrassing bug spotted) by: jake
f482b3c0f7