Fix membar positioning in mtx_enter_try() and (critically!) mtx_leave()
Fix membar positioning in mtx_enter_try() and (critically!) mtx_leave()

Same diff as guenther@ committed for alpha.
ok guenther@

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