Staging: batman-adv: Always protect list_for_each_entry_rcu with RCU

receive_bat_packet is not called with rcu_read_lock so we must ensure by
ourself that we protect list_for_each_entry_rcu using the correct RCU
locks.

Signed-off-by: Sven Eckelmann <sven.eckelmann@gmx.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>