[NETNS][IPV6]: Make sysctls route per namespace.

All the sysctl concerning the routes are moved to the network
namespace structure. A helper function is called to initialize the
variables.

Because the ipv6 protocol is not yet per namespace, the variables are
accessed relatively from the network namespace.

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