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/icmp4.c b/src/vnet/ip/icmp4.c
index a4808f2..1fe01e4 100644
--- a/src/vnet/ip/icmp4.c
+++ b/src/vnet/ip/icmp4.c
@@ -513,15 +513,13 @@
 		  b->current_length = 0;
 		}
 	    }
+	  p0->current_length =
+	    p0->current_length > 576 ? 576 : p0->current_length;
 
 	  /* Add IP header and ICMPv4 header including a 4 byte data field */
 	  vlib_buffer_advance (p0,
 			       -sizeof (ip4_header_t) -
 			       sizeof (icmp46_header_t) - 4);
-
-	  p0->current_length =
-	    p0->current_length > 576 ? 576 : p0->current_length;
-
 	  out_ip0 = vlib_buffer_get_current (p0);
 	  icmp0 = (icmp46_header_t *) & out_ip0[1];