Add membars to guarantee mtx_oldipl is written after locking and read before unlocking.  Believed to fix some spl problems on MP that have had landry and tobaisu seeing red.
Add membars to guarantee mtx_oldipl is written after locking and
read before unlocking.  Believed to fix some spl problems on MP
that have had landry and tobaisu seeing red.

suggestion to use membar API from matthew@
ok matthew@ kettenis@

orumin/openbsd-efivars,orumin/openbsd-efivars,orumin/openbsd-efivars,orumin/openbsd-efivars