>>>who makes the call
Either umpire is entitled to call 'No ball' in respect of an illegal bowling action (although I've only ever called 'No ball' from square leg just once myself - it's certainly uncommon).
Law 24 (3) states: "A ball is fairly delivered in respect of the arm if, once the bowler’s arm has reached the level of the shoulder in the delivery swing, the elbow joint is not straightened partially or completely from that point until the ball has left the hand. This definition shall not debar a bowler from flexing or rotating the wrist in the delivery swing".
Plenty of video evidence has shown that Lasith Mainga's action complies with the wording of that Law. A few other bowlers (notably Vinay Kumar) are now deliberately inserting a few Malinga-style deliveries among their more regular ones, often with devastating effects for the opposing batsmen.