[NETNS]: Use the per-net ipv6_devconf(_all) in sysctl handlers

Actually the net->ipv6.devconf_all can be used in a few places,
but to keep the /proc/sys/net/ipv6/conf/ sysctls work consistently
in the namespace we should use the per-net devconf_all in the
sysctl "forwarding" handler.

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