LONDON (Reuters) -Prime Minister Boris Johnson rejected calls on Monday for Britain to ease visa-demands on Ukrainian refugees fleeing conflict, saying Britain was a generous country but it needed to maintain checks on who was arriving. “We are a very, very generous country. What we want though is control and we want to be able to check,” he told reporters. “I think it’s sensible given what’s going on in Ukraine to make sure that we have some basic ability to check who is coming in.” “What we won’t do is have a system where people can come into the UK without any checks or any controls at all …