Can't learn Master Smithing from Eorlund Graymane?
No trainers can get you to level 100. See here. Quote: "Unlike Oblivion, there are no means to be trained all the way to level 100 in a skill." Other wiki sites agree, here and here.
90 is the maximum level you can get trained to by a Master trainer. After that, you have to do it yourself, by spamming weapons and/or armor in this case.