tapv2: Sporadic SIGABRT in ethernet_input [VPP-1183]

virtio_free_rx_buffers uses the wrong slot in the vring to get
the buffer index. It uses desc_next. It should be last_used_idx
which is the slot number for the first valid descriptor.

Change-Id: I6b62b794f06869fbffffce45430b8b2e37b1266c
Signed-off-by: Steven <sluong@cisco.com>
diff --git a/src/vnet/devices/tap/cli.c b/src/vnet/devices/tap/cli.c
index c17599a..9d86159 100644
--- a/src/vnet/devices/tap/cli.c
+++ b/src/vnet/devices/tap/cli.c
@@ -265,8 +265,9 @@
 	// RX = 0, TX = 1
 	vring = vec_elt_at_index (vif->vrings, i);
 	vlib_cli_output (vm, "  Virtqueue (%s)", (i & 1) ? "TX" : "RX");
-	vlib_cli_output (vm, "    qsz %d, last_used_idx %d, desc_in_use %d",
-			 vring->size, vring->last_used_idx,
+	vlib_cli_output (vm,
+			 "    qsz %d, last_used_idx %d, desc_next %d, desc_in_use %d",
+			 vring->size, vring->last_used_idx, vring->desc_next,
 			 vring->desc_in_use);
 	vlib_cli_output (vm,
 			 "    avail.flags 0x%x avail.idx %d used.flags 0x%x used.idx %d",