Ex-'Smallville' Star Allison Mack Pleads Guilty in Sex Cult Case

10:47 AM PT -- The U.S. Attorney's office tells TMZ Mack is facing up to 40 years, 20 for each count. Former "Smallville" star Allison Mack just entered a guilty plea to racketeering charges in her sex cult case ... and all signs point to her…