Following several years of detours, “The Curious Case of Benjamin Button,” “The Social Network,” David Fincher went back to one of his bread and butter obsessions, serial killers, and the procedural film with Netflix’s series “Mindhunter.” In a way riffing on things he had already done in “Zodiac,” “Mindhunter” was a continuation of his preoccupations with fixation, through the lens of serial killers, but reverse-engineered, by those that try and catch and understand them: the FBI.