[NETNS][IPV6]: Make multiple instance of sysctl tables.

Each network namespace wants its own set of sysctl value, eg. we
should not be able from a namespace to set a sysctl value for another
namespace , especially for the initial network namespace.

This patch duplicates the sysctl table when we register a new network
namespace for ipv6. The duplicated table are postfixed with the
"template" word to notify the developper the table is cloned.

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