I am reading Prof. Reinhold Trautmann's Die altpreußischen Personennamen (1925). What the professor writes about the suffix –in in the Old Prussian personal names is very interesting regarding the Tolkien family name.
If I understand well the Old Prussian –īn is a form of ablaut of the suffix –ain– / –ein– 'belonging to', 'son or descendant of'. If so, the family name Tolkien (< *Tolkain) means 'son or descendant of Tolk'. About the noble family Tolk von Merklingerode you can read here.