batman-adv: drop QinQ claim frames in bridge loop avoidance

Since bridge loop avoidance only supports untagged or simple 802.1q
tagged VLAN claim frames, claim frames with stacked VLAN headers (QinQ)
should be detected and dropped. Transporting the over the mesh may cause
problems on the receivers, or create bogus entries in the local tt
tables.

Reported-by: Antonio Quartulli <antonio@open-mesh.com>
Signed-off-by: Simon Wunderlich <simon@open-mesh.com>
Signed-off-by: Marek Lindner <mareklindner@neomailbox.ch>
Signed-off-by: Antonio Quartulli <antonio@meshcoding.com>