[NETNS][IPV6] Move sysctl initialization later on in the IPv6 init sequence

This patch moves initialization of IPv6 sysctl stuff at the end of
IPv6 initialization.

This will be helpful for network namespaces where some sysctl entries
depend on per-namespace variables, that need to be allocated and
initialized before they are referenced by sysctl.

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>