Add explicit membars, so that we can stop requiring rw_cas() to provide the magic.
Add explicit membars, so that we can stop requiring rw_cas() to
provide the magic.

ok matthew@ dlg@

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