Here some more math

Spray 75 bushel /acre oats with 0.5 kg of glyphosate per acre which is 500,000 mg so if all the glyphosate lands on the oat seed and all of it penetrates the hull and get adsorbed into the grain and none of it breaks down into something else, then you would have to eat 0.5856 pounds of oats to get 122 mg. 

(122/500,000) x 75 x 32 = 0.5856 pounds

I would guess that the percent of glyphosate that gets onto and stays on the seed without the hull to be a lot closer to 10% (probably less) of applied then it is to 100%. If it is 10% then a person would have to eat 5.856 lb of oats to get 122 mg.

LD 50 of glyphosate is 5600mg/kg, Sodium chloride is 2690mg/kg, caffeine is 150 mg/kg, paraquat 120 mg/kg

So glyphosate is twice as safe as salt, and paraquat is slightly more dangerous than caffeine. 

The LD 50 of glyphosate for a 65 kg person would be 364,000 mg or 25.6 fluid oz of 4 lb glyphosate.