In all cases of implied entailments, twenty-one years' actual possession of the premises and treatment of the estate as a fee simple, by the donee and those claiming through or under him by grant in form of conveyance, in fee simple or by judicial sale, shall be held a complete bar of such implied or supposed entailment: Provided, That such bar shall not operate against any inheritance that may have vested in any heir in tail before the passage of this act.