batman-adv: Avoid name based attaching of hard interfaces

The sysfs code for the batman-adv/mesh_iface file was receiving a string of
the batadv interface. This interface name was then provided to the code
which shared sysfs+rtnetlink code for attaching an hard-interface to an
batadv interface. The rtnetlink code was also using the (extracted)
interface name from the ndo_add_slave callback to increase the shared code
- even when it would have been more efficient to use the provided
net_device object directly instead of searching it again (based on its
name) in batadv_hardif_enable_interface.

But this indirect handling is no longer necessary because the sysfs code
was dropped. There is now only a single code path which is using
batadv_hardif_enable_interface.

Signed-off-by: Sven Eckelmann <sven@narfation.org>
Signed-off-by: Simon Wunderlich <sw@simonwunderlich.de>