accounting: fix memory leak on retries operations

Before this change if an operation was retried on operations.Copy and
the operation was large enough to use an async buffer then an async
buffer was leaked on the retry.  This leaked memory, a file handle and
a go routine.

After this change if Account.WithBuffer is called and there is already
a buffer, then a new one won't be allocated.

fs/accounting/accounting.go

@@ -70,6 +70,10 @@ func newAccountSizeName(stats *StatsInfo, in io.ReadCloser, size int64, name str
 
 // WithBuffer - If the file is above a certain size it adds an Async reader
 func (acc *Account) WithBuffer() *Account {
+	// if already have a buffer then just return
+	if acc.withBuf {
+		return acc
+	}
 	acc.withBuf = true
 	var buffers int
 	if acc.size >= int64(fs.Config.BufferSize) || acc.size == -1 {
@@ -118,14 +122,16 @@ func (acc *Account) StopBuffering() {
 // async buffer (if any) and re-adding it
 func (acc *Account) UpdateReader(in io.ReadCloser) {
 	acc.mu.Lock()
-	if acc.withBuf {
+	withBuf := acc.withBuf
+	if withBuf {
 		acc.StopBuffering()
+		acc.withBuf = false
 	}
 	acc.in = in
 	acc.close = in
 	acc.origIn = in
 	acc.closed = false
-	if acc.withBuf {
+	if withBuf {
 		acc.WithBuffer()
 	}
 	acc.mu.Unlock()