batman-adv: add B.A.T.M.A.N. Dump BLA claims via netlink

Dump the list of bridge loop avoidance claims via the netlink socket.

Signed-off-by: Andrew Lunn <andrew@lunn.ch>
[sven.eckelmann@open-mesh.com: add policy for attributes, fix includes, fix
soft_iface reference leak]
Signed-off-by: Sven Eckelmann <sven.eckelmann@open-mesh.com>
[sw@simonwunderlich.de: fix kerneldoc, fix error reporting]
Signed-off-by: Simon Wunderlich <sw@simonwunderlich.de>
Signed-off-by: Marek Lindner <mareklindner@neomailbox.ch>