I guess that the steel box of the fridge will have been damaged such that melt-water from the defrost process has penetrated the protective cosmetic covering and contacted the steel beneath which has rusted.
Aside from a warranty claim - or a new fridge - to get rid of the rust and prevent it returning will need the protective finish removed - the steel abraded to remove all the rust and a new protective coating applied - the fridge will need to be out of use for several days. It will probably not be practical to do this.
In the mean time try to keep food items from coming into contact with the affected parts - rust is not poisonous - iron is good for the blood so I believe.
Good Luck...
February 2016