openflow/learn_flow_spec: Fix to use src field if dst field is omitted, as dst field.

examples/nx_learning_switch/lib/nx_learning_switch.ex

@@ -56,29 +56,7 @@ defmodule NxLearningSwitch do
     send_flow_mod_add(
       datapath_id,
       table_id: 0,
-      instructions: ApplyActions.new([
-        NxLearn.new(
-          table_id: 1,
-          priority: 2,
-          hard_timeout: 10,
-          flow_specs: [
-            NxFlowSpecMatch.new(
-              src: :eth_src,
-              dst: :eth_dst
-            ),
-            NxFlowSpecMatch.new(
-              src: :vlan_vid,
-              dst: :vlan_vid,
-              offset: 0,
-              n_bits: 12
-            ),
-            NxFlowSpecOutput.new(
-              src: :nx_in_port
-            )
-          ]
-        ),
-        NxResubmitTable.new(1)
-      ])
+      instructions: l2_learning_instr()
     )
   end
 
@@ -90,4 +68,28 @@ defmodule NxLearningSwitch do
       instructions: ApplyActions.new(Output.new(:flood))
     )
   end
+
+  defp l2_learning_instr do
+    ApplyActions.new([
+      l2_learning_action(),
+      NxResubmitTable.new(1)
+    ])
+  end
+
+  defp l2_learning_action do
+    NxLearn.new(
+      table_id: 1,
+      priority: 2,
+      hard_timeout: 10,
+      flow_specs: l2_learning_flow_specs()
+    )
+  end
+
+  defp l2_learning_flow_specs do
+    [
+      NxFlowSpecMatch.new(src: :eth_src, dst: :eth_dst),
+      NxFlowSpecMatch.new(src: :vlan_vid, n_bits: 12),
+      NxFlowSpecOutput.new(src: :nx_in_port)
+    ]
+  end
 end