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.
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.