Get a jug of Zud powder or Bartender's Friend. They contain oxalic acid, which will dissolve rust. Works great on stained sinks, too. Find it next to the Comet cleanser at the grocery store.
Wet the stain. Pour a pile of Zud all over the stain, making sure it's completely dampened by the moisture and kinda pasty. Let it sit for an hour or so, then rinse it off.