batman-adv: Distributed ARP Table - create DHT helper functions

Add all the relevant functions in order to manage a Distributed Hash Table over
the B.A.T.M.A.N.-adv network. It will later be used to store several ARP entries
and implement DAT (Distributed ARP Table)

Signed-off-by: Antonio Quartulli <ordex@autistici.org>
diff --git a/net/batman-adv/unicast.h b/net/batman-adv/unicast.h
index a88ed29..61abba5 100644
--- a/net/batman-adv/unicast.h
+++ b/net/batman-adv/unicast.h
@@ -32,6 +32,10 @@
 int batadv_frag_send_skb(struct sk_buff *skb, struct batadv_priv *bat_priv,
 			 struct batadv_hard_iface *hard_iface,
 			 const uint8_t dstaddr[]);
+bool batadv_unicast_4addr_prepare_skb(struct batadv_priv *bat_priv,
+				      struct sk_buff *skb,
+				      struct batadv_orig_node *orig_node,
+				      int packet_subtype);
 int batadv_unicast_generic_send_skb(struct batadv_priv *bat_priv,
 				    struct sk_buff *skb, int packet_type,
 				    int packet_subtype);