[NETNS][IPV6] route6 - move ip6_dst_ops inside the network namespace

The ip6_dst_ops is moved inside the network namespace structure.  All
references to this structure are now relative to the initial network
namespace.

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