be in {pmap_pinit, pmap_copy, pmap_release} at a time.
This reduces the rate of panics when running 'make index' from ~0.6/hour
to ~0.02/hour (p < 10^-30).
At a later date this locking will be removed, and for this reason, it is
wrapped in #ifdef HAMFISTED_LOCKING; this temporary hack is being put in
place with the intention of shipping somewhat-stable Xen bits in FreeBSD
8.2-RELEASE.
PR: kern/153672
MFC after: 3 days
aa829cef6a