Actually man, I asked this in a thread a while back because i was curious too. Did some research, Howlin' Wolf did a album in the 60's remaking his own songs. Giving them a pshycadelic fell to them. That riff is in the redone version of Smokestack Lightning. Now, i'm not sure if that is where Page got it from because back then riffs repeated so much, hard telling where it came from, but note for note, that's where i found it.