The big leagues aren’t always kind when teaching lessons to young players. Luckily, Las Vegas Aviators pitcher Mason Barnett has a great mentor in his corner. Tim Hudson — a member of the Athletics Hall of Fame with 222 big league wins — was Barnett’s pitching coach at Auburn. The two old-school, hard-nosed pitchers keep in touch, with Hudson offering advice, kudos and encouragement. “Having him at Auburn opened my mind to how pro ball would work,” Barnett said. “It definitely helped me prepare once I took that next step.” Like Hudson, Barnett is a steely competitor with a Southern drawl and small-town Georgia roots. The A’s are hoping he can deliver like their former ace, too. If Barnett — the Athletics’ No. 17 prospect per MLB Pipeline — keeps pitching the way he has been, it’s only a matter of time before he rejoins the A’s. This year, the righty is 2-0 with a 3.07 ERA and second on the team with 32 strikeouts in 29⅓ innings. In Thursday’s 7-6 win over the visiting St.…