batman-adv: store hard_iface as iflink private data

By passing the hard_iface to netdev_master_upper_dev_link() as private
data, we can iterate over hardifs of a mesh interface more efficiently
using netdev_for_each_lower_private*() (instead of iterating over the
global hardif list). In addition, this will enable resolving a hardif
from its netdev using netdev_lower_dev_get_private() and getting rid of
the global list altogether in the following patches.

A similar approach can be seen in the bonding driver.

Signed-off-by: Matthias Schiffer <mschiffer@universe-factory.net>
Signed-off-by: Sven Eckelmann <sven@narfation.org>
Signed-off-by: Simon Wunderlich <sw@simonwunderlich.de>