By Alastair Macdonald VALLETTA (Reuters) - European Union leaders agreed on Thursday to invite Turkish President Tayyip Erdogan to a summit very soon as they seek his help to stem a chaotic flow of migrants that threatens Europe's unity and open borders. Donald Tusk, the European Council president who chaired the emergency meeting of EU leaders on Malta, warned that they must win a "race against time" to slow arrivals via Greece if Europe is to save the Schengen zone of passport-free travel inside the bloc from being throttled by new national barriers and controls. At the meeting, which followed a summit on the same topic with African counterparts, leaders were briefed on negotiations with Ankara the EU executive launched a month ago and gave the green light to wrapping them up.