Atmega328p (For the love of satan, someone got a clue?)

Projects Programming
5

From the reference:

attachInterrupt(digitalPinToInterrupt(pin), ISR, mode);	(recommended)

So

    attachInterrupt(digitalPinToInterrupt(FS_PIN_1), FanSpeed_RPM, RISING);

would be the best way to code this, but doesn't help you discover that pin 9 isn't an interrupt pin...