That's why they walked through it. However, I feel like it was said somewhere that Dragon fire can kill the White Walkers.

>Fire-- **paging Drogon and his bros** BUT....them niggas
>walked thru fire a few eps back ....which is why I gave them a
>.5

So the Dragons coming back, all the red magic, Jon's ascension to the crown is all really happening as a reaction to Winter coming in preparation for the battle with the White Walkers. There's been tons of talk about magic getting stronger every which way...it's basically a yin and yang situation. They have the pieces to fight them off.