batman-adv: NETIF_F_NETNS_LOCAL feature to prevent netns moves

The batX soft interface should not be moved between network name
spaces. This is similar to bridges, bonds, tunnels, which are not
allowed to move between network namespaces.

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