[NETNS][IPV6] tcp - assign the netns for timewait sockets

Copy the network namespace from the socket to the timewait socket.

Signed-off-by: Daniel Lezcano <dlezcano@fr.ibm.com>
Acked-by: Mark Lord <mlord@pobox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>