The difference between 22.2mm and 21.7mm is .5mm or .020". On a cyclindrical workpiece, that would be a cut of .010". Easy-peasey work for a metal lathe but a lot of sanding by hand, and hand sanding would likely result in an out-of-round part. Problem with doing it in a lathe is that the stem is an odd shape so is hard to hold in the lathe chuck without special fixturing and setup. A good machinist could do it but would probably charge more that you'd want to spend unless he were a hobbyist who really got interested in the project. Machine shop time is billed at about $100 and hour. Might be easier to find a shop that can ream the steerer tube with an adjustable parallel reamer.