[mesh-forwarder] add log to RemoveMessage() (#2198)

src/core/thread/mesh_forwarder.cpp

@@ -284,6 +284,7 @@ void MeshForwarder::RemoveMessage(Message &aMessage)
     }
 
     mSendQueue.Dequeue(aMessage);
+    LogIp6Message(kMessageEvict, aMessage, NULL, OT_ERROR_NO_BUFS);
     aMessage.Free();
 }
 
@@ -2437,6 +2438,10 @@ void MeshForwarder::LogIp6Message(MessageAction aAction, const Message &aMessage
         actionText = "Dropping (reassembly timeout)";
         break;
 
+    case kMessageEvict:
+        actionText = "Evicting";
+        break;
+
     default:
         actionText = "";
         break;

src/core/thread/mesh_forwarder.hpp

@@ -260,6 +260,7 @@ private:
         kMessagePrepareIndirect,         ///< Indicates that the message is being prepared for indirect tx.
         kMessageDrop,                    ///< Indicates that the outbound message is being dropped (e.g., dst unknown).
         kMessageReassemblyDrop,          ///< Indicates that the message is being dropped from reassembly list.
+        kMessageEvict,                   ///< Indicates that the message was evicted.
     };
 
     otError CheckReachability(uint8_t *aFrame, uint8_t aFrameLength,