Revert "MTU: Setting of MTU on software interface (instead of hardware interface)"

This reverts commit 70083ee74c3141bbefb185525315f1b34497dcaa.

Reverting as this patch is causing following crash:

0: /home/damarion/cisco/vpp3/build-data/../src/vnet/devices/devices.h:131 (vnet_get_device_input_thread_index) assertion `queue_id < vec_len (hw->input_node_thread_index_by_queue)' fails
Aborted

Change-Id: Ie2a365032110b1f67be7a9d832885b9899813d39
Signed-off-by: Damjan Marion <damarion@cisco.com>
diff --git a/src/vnet/ip/ip6_neighbor.c b/src/vnet/ip/ip6_neighbor.c
index a1439fa..fee4356 100644
--- a/src/vnet/ip/ip6_neighbor.c
+++ b/src/vnet/ip/ip6_neighbor.c
@@ -2575,6 +2575,10 @@
     {
       if (is_add)
 	{
+	  vnet_hw_interface_t *hw_if0;
+
+	  hw_if0 = vnet_get_sup_hw_interface (vnm, sw_if_index);
+
 	  pool_get (nm->if_radv_pool, a);
 
 	  ri = a - nm->if_radv_pool;
@@ -2608,8 +2612,7 @@
 	  a->send_radv = 1;
 
 	  /* fill in radv_info for this interface that will be needed later */
-	  a->adv_link_mtu =
-	    vnet_sw_interface_get_mtu (vnm, sw_if_index, VLIB_TX);
+	  a->adv_link_mtu = hw_if0->max_l3_packet_bytes[VLIB_RX];
 
 	  clib_memcpy (a->link_layer_address, eth_if0->address, 6);