By Andrew OsbornMOSCOW, March 3 (Reuters) - When President Donald Trump returned to the White House last year, some Russian hardliners were cautiously optimistic, hoping his unpredictability and transactional nature might benefit Moscow on Ukraine.But his attack on Iran means many now see him as a growing threat to Russia itself and are questioning if Trump is the pragmatic, potentially pro-Moscow strongman ready to deal in realpolitik that they thought he was.