Always retain the route in the tcp and fake transport endpoints on connect

**Description**

Currently the tcp endpoint and [fakeTransportEndpoint](https://cs.opensource.google/gvisor/gvisor/+/master:pkg/tcpip/stack/transport_test.go;l=149?ss=gvisor) release the route on connect rather than retaining it. Since multiple connect calls are permitted, it might make sense to retain the route rather than releasing it every time.