“ Masterpiece ” hit “ All Creatures Great and Small ,” which returns for Season 6 in January on PBS , has already been renewed for a Season 7 and a Season 8. Those additional seasons will include six episodes each, as well as additional Christmas specials. The news was announced Thursday by “Masterpiece” commissioner GBH. Playground (“Wolf Hall: The Mirror and the Light,” “The Hardacres”) produces “All Creatures Great and Small,” along with “Masterpiece” on PBS and global distributor All3Media