When connections go to same node and deadlock happens, BF abort should
not happen for victim thread. Fixed by guarding
`wsrep_handle_SR_rollback()` so that is called only for SR transactions.
Co-authored-by: Seppo Jaakola <seppo.jaakola@iki.fi>
Co-authored-by: Daniele Sciascia <daniele.sciascia@galeracluster.com>