[muxorch] Reorder the neighbor disable operations (sonic-net#2917)

When mux transits from active to standby, orchagent first disable the neighbor[1], then program the tunnel route[2].
There is a small period of time between [1] and [2] that the ASIC is unable to forward the traffic to the neighbor.

So let's reorder those two operations - program the tunnel route first, then disable the neighbor.

orchagent/muxorch.cpp

@@ -786,13 +786,6 @@ bool MuxNbrHandler::disable(sai_object_id_t tnh)
             return false;
         }
 
-        neigh = NeighborEntry(it->first, alias_);
-        if (!gNeighOrch->disableNeighbor(neigh))
-        {
-            SWSS_LOG_INFO("Disabling neigh failed for %s", neigh.ip_address.to_string().c_str());
-            return false;
-        }
-
         updateTunnelRoute(nh_key, true);
 
         IpPrefix pfx = it->first.to_string();
@@ -801,6 +794,13 @@ bool MuxNbrHandler::disable(sai_object_id_t tnh)
             return false;
         }
 
+        neigh = NeighborEntry(it->first, alias_);
+        if (!gNeighOrch->disableNeighbor(neigh))
+        {
+            SWSS_LOG_INFO("Disabling neigh failed for %s", neigh.ip_address.to_string().c_str());
+            return false;
+        }
+
         it++;
     }