AF_XDP-interaction: Fix function tx_pkt

If there were more transmit slots, then we umem free the
packet, but we continued sending it anyhow.

The places tx_pkt() is currently used this never happened.
Still fix the bug.

Signed-off-by: Jesper Dangaard Brouer <brouer@redhat.com>

Author: netoptimizer

AF_XDP-interaction/af_xdp_user.c

@@ -884,8 +884,9 @@ static int tx_pkt(struct config *cfg, struct xsk_socket_info *xsk)
 		if (ret != 1) {
 			/* No more transmit slots, drop the packet */
 			mem_free_umem_frame(&umem->mem, pkt_addr);
-			fprintf(stderr, "ERR - %s() failed transmit\n",
+			fprintf(stderr, "ERR: %s() failed transmit, no slots\n",
 				__func__);
+			return ENOSPC;
 		}
 
 		xsk_ring_prod__tx_desc(&xsk->tx, tx_idx)->addr = pkt_addr;