Kellari is very likely to have been borrowed from the Swedish "källare". It's not the other way round since the Swedish källare has well-known cognates in the other Germanic languages (Kellner in German, kjeller in Norwegian, cellar in English, etc.) and non-Germanic (cellier in French, celler in Catalan, etc.).
Basement - kellarikerros, cellar - kellari. Finns are not storing their food under their houses, never have not even in the stone age and kellari means only the food storing place. Kellarikerros means rooms under the house where can be garage, tv-room, rooms for hobbies, or for just about any use, even a boiler room. :)