batman-adv: Suppress debugfs entries for netns's

Debugfs is not netns aware. It thus has problems when the same
interface name exists in multiple network name spaces.

Work around this by not creating entries for interfaces in name spaces
other than the default name space. This means meshes in network
namespaces cannot be managed via debugfs, but there will soon be a
netlink interface which is netns aware.

Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Simon Wunderlich <sw@simonwunderlich.de>
Signed-off-by: Sven Eckelmann <sven@narfation.org>
Signed-off-by: Marek Lindner <mareklindner@neomailbox.ch>