ieee802154: allow netns create of lowpan interface

This patch reverts commit f9d1ce8f ("ieee802154: fix netns settings").
The lowpan interface need to be created inside the net namespace where
the wpan interface is available. The wpan namespace can be changed only
by nl802154 before. Without this patch it's not possible to create a
lowpan interface for a wpan interface which isn't inside init_net
namespace.

Cc: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Reviewed-by: Stefan Schmidt <stefan@osg.samsung.com>
Signed-off-by: Alexander Aring <aar@pengutronix.de>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>