How much has this bike been ridden in the rain? Are you its original owner? How old is it? Personally, I wouldn't be overly concerned about this little bit of rust unless your noticed water dripping out of that hole. If water is getting into the top of the fork and dripping out, then the entire inside of the fork may have alot of rust. Otherwise, I'd suspect that the rust if fairly localized and due to rain or humidity entering the little holes. There shouldn't be an opening for rain to enter anywhere else. You could remove the stem to look inside the steerer tube for signs of rust or water entry. But the fork blades are actually separate pieces of metal that shouldn't have a connection to the steerer tube. They should be sealed except for the little hole that allows air pressure to equalize. I assume that it couldn't hurt to dump some Boshield or other rust inhibitor in there, but don't try to seal the holes.