You can't "really" fix it, adding rubber to the tire is not an option. The pain has already been inflicted.
Safety is ALWAYS my #1 concern. So I would just buy a new tire and sleep better at night while having the best possible confidence in my bike's tires in all conditions.
If not.... What would "I" do in your shoes?
Step 1 - Stop it from rubbing.
Do you have plenty of chain slack? If so, adjusting things back a mm or two may cure the rubbing problem. As long as your chain still has the proper slack after moving it and a 107 link chain is an option fyi. As you ride, the line will wear itself out. ie. heal itself via raw milage.
Sell the tire as a "take off" and buy a more fitting size. If you go this route, do it NOW! Before the line gets too deep. A new rear tire is cheap enough but if I couldn't sell the larger tire, I would replace it anyway and store it in the garage for a later time or until it does sell.
I look at it like this, it's pretty much the same if you had a more "routine" issue with a new tire; a nail, a screw, a manufacturing defect, won't hold air well, whatever... If it needs replaced, then replace it and chalk it up to the cost of doing business. We ALL make mistakes that cost $$ sometimes. As riders, we forever will need to eval our own risk in tires and what we are willing to "bet our arses" riding on. It's the T in tclocs remember?
Some riders will ride happily on that tire, others... no so much.
Adding a link to the chain is considered taboo at best, please DO NOT be tempted to do this. "Adding" risk elements should be avoided at all costs, and you just be replacing one with another.