Finally protect VP lookups to guarantee that a pted won't be freed or

reused by a CPU while another CPU is manipulating it.

This races occurs because the virtual spill handlers are run without
taking the KERNEL_LOCK for obvious reasons.  So use a per-pmap mutex
that CPUs must hold when modifying a pted in order to guarantee the
atomicity of operations *and* the coherence between pmap VPs tree and
what's in the HASH.

Thanks to dlg@ for assisting me debugging this.  This change ends your
PowerPC pmap SMP show of the week.  GENERIC.MP on macppc should now be
stable enough to build ports without corrupting its own memory.

ok kettenis@, deraadt@, dlg@

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