If the Jets want Eric Mangini as their next head coach, the feeling is very mutual, The Post learned exclusively last night. Although Bill Belichick has been trying to talk Mangini out of taking the Jets job, Mangini will make his own decision, and that decision would be to accept a fair Jets' offer if one is, indeed, made. The Jets' heavy-breathing interest in the Patriots' defensive coordinator and Belichick disciple is one of the worst-kept secrets in the NFL. The Jets are scheduled to interview Mangini tomorrow near Foxborough, Mass., and he's believed to be at the top of the team's wish list. The level of Mangini's interest in the Jets has been a question of significance that's been raised in recent days.