ipv6: don't use tw net when accounting for recycled tw

We already have a valid net in that place, but this is not just a
cleanup - the tw pointer can be NULL there sometimes, thus causing
an oops in NET_NS=y case.

The same place in ipv4 code already works correctly using existing 
net, rather than tw's one.

The bug exists since 2.6.27.

Signed-off-by: Pavel Emelyanov <xemul@openvz.org>
Signed-off-by: David S. Miller <davem@davemloft.net>