batman-adv: Set special lockdep classes to avoid lockdep warning

Transmissions over batman-adv devices always start another nested transmission
over devices attached to the batman-adv interface. These devices usually use
the ethernet lockdep class for the tx_queue lock which is also set by default
for all batman-adv devices. Lockdep will detect a nested locking attempt of two
locks with the same class and warn about a possible deadlock.

This is the default and expected behavior and should not alarm the locking
correctness prove mechanism. Therefore, the locks for all netdevice specific tx
queues get a special batman-adv lock class to avoid a false positive for each
transmission.

Reported-by: Linus Luessing <linus.luessing@web.de>
Signed-off-by: Sven Eckelmann <sven@narfation.org>
Signed-off-by: Antonio Quartulli <ordex@autistici.org>