batman-adv: check return type of unicast packet preparations

batadv_unicast(_4addr)_prepare_skb  might reallocate the skb's data.
And if it tries to do so then this can potentially fail.

We shouldn't continue working on this skb in such a case.

Signed-off-by: Linus Lüssing <linus.luessing@web.de>
Signed-off-by: Marek Lindner <lindner_marek@yahoo.de>
Acked-by: Antonio Quartulli <ordex@autistici.org>
Signed-off-by: Antonio Quartulli <ordex@autistici.org>

net/batman-adv/unicast.c

@@ -432,12 +432,16 @@ find_router:
 
 	switch (packet_type) {
 	case BATADV_UNICAST:
-		batadv_unicast_prepare_skb(skb, orig_node);
+		if (!batadv_unicast_prepare_skb(skb, orig_node))
+			goto out;
+
 		header_len = sizeof(struct batadv_unicast_packet);
 		break;
 	case BATADV_UNICAST_4ADDR:
-		batadv_unicast_4addr_prepare_skb(bat_priv, skb, orig_node,
-						 packet_subtype);
+		if (!batadv_unicast_4addr_prepare_skb(bat_priv, skb, orig_node,
+						      packet_subtype))
+			goto out;
+
 		header_len = sizeof(struct batadv_unicast_4addr_packet);
 		break;
 	default: