batman-adv: Check hard_iface refcnt when receiving skb

The receive function may start processing an incoming packet while the
hard_iface is shut down in a different context. All called functions called
with the batadv_hard_iface object belonging to the incoming interface would
have to check whether the reference counter is still > 0.

This is rather error-prone because this check can be forgotten easily.
Instead check the reference counter when receiving the object to make sure
that all called functions have a valid reference.

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