By Valentina Za and Giuseppe Fonte MILAN (Reuters) -Monte dei Paschi di Siena was racing on Wednesday to get commitments from investors for its 2.5 billion euro ($2.4 billion) share issue so it can secure a backstop from banks for any unsold stock, three people close to the matter said. With markets gripped by fears about recession, conflict in Ukraine, inflation and higher rates, the group of banks due to underwrite the MPS sale have refused to take on the risk without reassurances about how much stock they could be left holding. Five years after an 8.2 billion euro ($8 billion) bailout that …