eth/fetcher: add missing timer.Stop calls (#20861)

eth/fetcher/block_fetcher.go

@@ -302,6 +302,8 @@ func (f *BlockFetcher) loop() {
 	// Iterate the block fetching until a quit is requested
 	fetchTimer := time.NewTimer(0)
 	completeTimer := time.NewTimer(0)
+	defer fetchTimer.Stop()
+	defer completeTimer.Stop()
 
 	for {
 		// Clean up any expired block fetches