[NETNS]: Use the per-net ipv6_devconf_dflt

All its users are in net/ipv6/addrconf.c's sysctl handlers.
Since they already have the struct net to get from, the
per-net ipv6_devconf_dflt can already be used.

Signed-off-by: Pavel Emelyanov <xemul@openvz.org>
Signed-off-by: David S. Miller <davem@davemloft.net>