Rework disklabel reading taskq to plug holes pointed out by jsing@.
Rework disklabel reading taskq to plug holes pointed out by jsing@.

Diff from dlg@.

ok jsing@

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