KALAK, Iraq — “For the sake of the sacrifices and blood of the martyrs, let’s all say yes for Kurdistan independence,” reads a large billboard in the center of Kalak, a small town in Iraq’s northern Kurdish region. “Independence is not given, it’s taken!” reads another banner hanging below a cluster of red, green, yellow and white Kurdish flags. Iraq’s Kurds are set to vote Monday in a referendum on support for independence that has raised fears of instability across the region as the war against the Islamic State group winds down.