batman-adv: free skb on TVLV parsing success

When the TVLV parsing routine succeed the skb is left
untouched thus leading to a memory leak.

Fix this by consuming the skb in case of success.

Introduced by ef261577
("batman-adv: tvlv - basic infrastructure")

Reported-by: Russel Senior <russell@personaltelco.net>
Signed-off-by: Antonio Quartulli <antonio@open-mesh.com>
Tested-by: Russell Senior <russell@personaltelco.net>
Signed-off-by: Marek Lindner <mareklindner@neomailbox.ch>