batman-adv: Use kref_get for hard_iface subfunctions

The callers of the functions using batadv_hard_iface objects already make
sure that they hold a valid reference. The subfunctions don't have
to check whether the reference counter is > 0 because this was checked by
the callers.

The kref_get function instead WARNs (with debug information) when the
reference counter would still be 0. This makes a bug in batman-adv better
visible because kref_get_unless_zero would have ignored this problem.

Signed-off-by: Sven Eckelmann <sven@narfation.org>
Signed-off-by: Marek Lindner <mareklindner@neomailbox.ch>
Signed-off-by: Antonio Quartulli <a@unstable.cc>