batman-adv: adopt netdev_hold() / netdev_put()

Add a device tracker to struct batadv_hard_iface to help
debugging of network device refcount imbalances.

Signed-off-by: Eric Dumazet <edumazet@google.com>
[sven@narfation.org: fix kernel-doc, adopt for softif reference]
Signed-off-by: Sven Eckelmann <sven@narfation.org>
Signed-off-by: Simon Wunderlich <sw@simonwunderlich.de>