The ipv6_devconf_(all) and ipv6_devconf_dflt are currently
global, but should be per-namespace.

This set moves them on the struct net. Or, more precisely,
on the struct netns_ipv6, which is already added.

Unfortunately, many code in the ipv6 cannot yet provide a 
correct struct net to get the ipv6_devconf from (e.g. routing 
code), so this part of job is to be done after the appropriate 
parts are virtualized.

However, after this set user can play with the ipv6_devconf 
inside a namespace not affecting the others.

Signed-off-by: Pavel Emelyanov <xemul@openvz.org>