net: tcp: Avoid double acknowlegding retransmitted data

In the FIN_WAIT_1 any incoming data is dropped, but anknowledged. Add a
check to see if the data is already acknowleged to prevent double
acknowledging of the data and bringing the acknowlegde counter out of
sync. When the acknowledge counter gets out of sync, the connection will
never properly terminate any more.

Signed-off-by: Sjors Hettinga <s.a.hettinga@gmail.com>