[NETNS][IPV6]: Make icmpv6_time sysctl per namespace.

This patch moves the icmpv6_time sysctl to the network namespace
structure.

Because the ipv6 protocol is not yet per namespace, the variable is
accessed relatively to the initial network namespace.

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