batman-adv: Create batman soft interfaces within correct netns.

When creating a soft interface, create it in the same netns as the
hard interface. Replace all references to init_net with the correct
name space for the interface being manipulated.

Suggested-by: Daniel Ehlers <danielehlers@mindeye.net>
Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Acked-by: Antonio Quartulli <a@unstable.cc>
Signed-off-by: Sven Eckelmann <sven@narfation.org>
Signed-off-by: Marek Lindner <mareklindner@neomailbox.ch>
Signed-off-by: Antonio Quartulli <a@unstable.cc>