MacCache.AddForced needs to return conflicting peer

otherwise NetworkRouter.handleForwardedPacket fails to clear out the
now invalid flows.

router/mac_cache.go

@@ -51,19 +51,21 @@ func (cache *MacCache) add(mac net.HardwareAddr, peer *mesh.Peer, force bool) (b
 		return true, nil
 	}
 
+	var conflictPeer *mesh.Peer
+
 	if entry.peer != peer {
+		conflictPeer = entry.peer
 		if !force {
-			return false, entry.peer
+			return false, conflictPeer
 		}
-
 		entry.peer = peer
 	}
 
 	if now.After(entry.lastSeen.Add(cache.maxAge / 10)) {
 		entry.lastSeen = now
 	}
 
-	return false, nil
+	return false, conflictPeer
 }
 
 func (cache *MacCache) Add(mac net.HardwareAddr, peer *mesh.Peer) (bool, *mesh.Peer) {