ieee802154: fix netns settings

6LoWPAN currently doesn't supports x-netns and works only in init_net.

With this patch, we ensure that:
 - the wpan interface cannot be moved to another netns;
 - the 6lowpan interface cannot be moved to another netns;
 - the wpan interface is in the same netns than the 6lowpan interface;
 - the 6lowpan interface is in init_net.

Signed-off-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Acked-by: Alexander Aring <alex.aring@gmail.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>