By Michelle Nichols UNITED NATIONS (Reuters) -U.N. aid chief Martin Griffiths “had frank and constructive discussions” with Russian officials in Moscow on facilitating exports of Ukraine grain from Black Sea ports, a U.N. spokesman said on Friday. Griffiths met with Russian foreign affairs and defense officials on Thursday and Friday, U.N. spokesman Stephane Dujarric said. Since Russia’s Feb. 24 invasion of Ukraine, Ukrainian grain shipments from its Black Sea ports have stalled and more than 20 million tonnes of grain are stuck in silos. “We’ve said clearly what we can do and what we cannot d…