This is not the way it works in 3e.
The barbarian rage is an effect that ends after (3+con mod) rounds, when the barbarian voluntarily ends it, or when certain spells end it (such as calm emotions).
Being knocked unconcious is not one of the conditions that ends the rage prematurely. Ruling that it does just makes Rage a suicidal tactic at higher levels.
If you want an "in character" rationalization for why it doesn't end, the hormones and adrenaline that fuel the rage are still in your bloodstream and won't dissipate for (3+con mod) rounds.