[NETNS][IPV6] tcp6 - make socket control per namespace

Instead of having a tcp6_socket global to all the namespace, there is
tcp6 socket control per namespace. That is consistent with which
namespace sent a RST and allows to pass the socket to the underlying
function to retrieve the network namespace.

Signed-off-by: Daniel Lezcano <dlezcano@fr.ibm.com>
Signed-off-by: Benjamin Thery <benjamin.thery@bull.net>
Signed-off-by: David S. Miller <davem@davemloft.net>