tests/net: Switch iface test to new net_pkt API

No need to play with net_buf by ourselves.

Signed-off-by: Tomasz Bursztyka <tomasz.bursztyka@linux.intel.com>
diff --git a/tests/net/iface/src/main.c b/tests/net/iface/src/main.c
index 3e0db4c..2635b23 100644
--- a/tests/net/iface/src/main.c
+++ b/tests/net/iface/src/main.c
@@ -109,7 +109,7 @@
 
 static int sender_iface(struct device *dev, struct net_pkt *pkt)
 {
-	if (!pkt->frags) {
+	if (!pkt->buffer) {
 		DBG("No data to send!\n");
 		return -ENODATA;
 	}
@@ -420,10 +420,11 @@
 	struct net_pkt *pkt;
 	int ret;
 
-	pkt = net_pkt_get_reserve_tx(K_FOREVER);
-	net_pkt_set_iface(pkt, iface);
+	pkt = net_pkt_alloc_with_buffer(iface, sizeof(data),
+					AF_UNSPEC, 0, K_FOREVER);
 
-	net_pkt_append_all(pkt, sizeof(data), data, K_FOREVER);
+	net_pkt_write_new(pkt, data, sizeof(data));
+	net_pkt_cursor_init(pkt);
 
 	ret = net_send_data(pkt);
 	if (!expect_fail && ret < 0) {