netstack/tcp: Add LastAck transition.
netstack/tcp: Add LastAck transition.

Add missing state transition to LastAck, which should happen when the
endpoint has already recieved a FIN from the remote side, and is
sending its own FIN.

PiperOrigin-RevId: 265568314

google/gvisor,google/gvisor,google/gvisor,google/gvisor,google/gvisor,google/gvisor,google/gvisor,google/gvisor