Lightly crush coriander and cumin seeds with the mortar and pestle. Transfer to a large skillet, add oil and cook over high heat, stirring frequently, until fragrant, about 1 minute. Add chile flakes and cook until the spices smell toasty, 5 to 10 seconds. Remove from heat and add potatoes. Gently stir to coat, then gradually stir in the pepper mixture. Transfer to the prepared baking sheet, spreading the potatoes in an even layer.