net/ethernet/arp: Let ethernet L2 managing pkt's reference while sending

prepare_arp() was unreferencing original pkt (called pending there) in
case of error.

net_prepare_arp() was always unreferencing pkt, though it could have
been already unreferenced by prepare_arp() as seen previously which is
an extra bogus unref in this case.

And in case it returned NULL, ethernet_send() would return NET_DROP
which in turn would make net_if's tx code to unref again the pkt.

This patch ensures pkt is unrefed only once and at the right place.

Signed-off-by: Tomasz Bursztyka <tomasz.bursztyka@linux.intel.com>

subsys/net/ip/l2/arp.c

@@ -110,12 +110,13 @@ static inline struct net_pkt *prepare_arp(struct net_if *iface,
 
 	pkt = net_pkt_get_reserve_tx(sizeof(struct net_eth_hdr), K_FOREVER);
 	if (!pkt) {
-		goto fail;
+		return NULL;
 	}
 
 	frag = net_pkt_get_frag(pkt, K_FOREVER);
 	if (!frag) {
-		goto fail;
+		net_pkt_unref(pkt);
+		return NULL;
 	}
 
 	net_pkt_frag_add(pkt, frag);
@@ -176,11 +177,6 @@ static inline struct net_pkt *prepare_arp(struct net_if *iface,
 	net_buf_add(frag, sizeof(struct net_arp_hdr));
 
 	return pkt;
-
-fail:
-	net_pkt_unref(pkt);
-	net_pkt_unref(pending);
-	return NULL;
 }
 
 struct net_pkt *net_arp_prepare(struct net_pkt *pkt)
@@ -260,8 +256,6 @@ struct net_pkt *net_arp_prepare(struct net_pkt *pkt)
 						  addr, NULL, pkt);
 				NET_DBG("Resending ARP %p", req);
 
-				net_pkt_unref(pkt);
-
 				return req;
 			}
 

subsys/net/ip/l2/ethernet.c

@@ -203,6 +203,11 @@ static enum net_verdict ethernet_send(struct net_if *iface,
 		NET_DBG("Sending arp pkt %p (orig %p) to iface %p",
 			arp_pkt, pkt, iface);
 
+		if (pkt != arp_pkt) {
+			/* pkt went to ARP pending queue */
+			net_pkt_unref(pkt);
+		}
+
 		pkt = arp_pkt;
 
 		net_pkt_ll_src(pkt)->addr = (u8_t *)&NET_ETH_HDR(pkt)->src;