* Burt Reynolds has said that his real forte is directing, rather than acting , but he has yet to prove this in practice. It's not a question of technical skill: There's plenty of this in Sharky's Machine, his latest picture as filmmaker and star. It's a matter of attitude - of deciding to spend one's skill on positive projects. On the surface, ''Sharky's Machine,'' about a vice-squad policeman tracking down a vicious murderer, is a commonplace stew of sex and violence, punctuated with the antifemale slant that's common in today's movies. Just below the surface, there's an undercurrent of real viciousness, culminating in the most sadistic climax of any Hollywood thriller in quite a while. Reynolds's real talent as a director means little as long as he squanders it on such nasty stuff, which may well alienate more viewers than it titillates.