You are exhausing the space of outbound ports by opening that many outbound sockets within the TIME_WAIT period of two minutes. The first question you should ask yourself is does this represent a realistic load test at all? Is a real client really going to do that? If not, you just need to revise your testing methodology.
BTW SO_LINGER is the number of seconds the application will wait during close() for data to be flushed. It is normally zero. The port will hang around for the TIME_WAIT interval anyway if this is the end that issued the close. This is not the same thing. It is possible to abuse the SO_LINGER option to patch the problem. However that will also cause exceptional behaviour at the peer and again this is not the purpose of a test.