batman-adv: Indicate netlink socket can be used with netns.

Set the netnsof flag on the family structure, indicating it can
be used with different network name spaces.

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>