Release mutex before blocking during TCP handshake route resolution.

Without releasing the mutex, operations on the endpoint following a
nonblocking connect will not make progress until connect is complete.

PiperOrigin-RevId: 340467654

pkg/tcpip/transport/tcp/connect.go

@@ -496,7 +496,9 @@ func (h *handshake) resolveRoute() *tcpip.Error {
 		}
 
 		// Wait for notification.
+		h.ep.mu.Unlock()
 		index, _ = s.Fetch(true /* block */)
+		h.ep.mu.Lock()
 	}
 }