When code from r254064 in pmap_ts_referenced() drops pv lock and
blocks on a pmap lock, pmap_release() might proceed in parallel and
destroy the pmap mutex, since unlocked pv lock allows to remove pv
entry owned by the pmap.

For now, gate the pmap_release() on write-locked pvh_global_lock.
Since pmap_ts_release() does not unlock the global lock,
pmap_release() would not destroy pmap mutex until the
pmap_ts_referenced() finished.  We cannot enter pmap_ts_referenced()
and encounter a pv entry for the destroyed pmap if pmap_release()
passed the global lock gate, since pmap_remove_pages() would finish
earlier.

Reported by:	jeff, pho
Reviewed by:	alc
Tested by:	pho
Sponsored by:	The FreeBSD Foundation