accounting: fix file handle leak on errors - fixes #3547

In 53a1a0e3ef we introduced a problem where if there was an
error on the file being transferred then the file was re-opened and
the old one wasn't closed.

This was partially fixed in bfbddab46b however this didn't
address the case of the old file being closed.

This is now fixed by
- marking the file as open again in UpdateReader
- moving the stopping the accounting machinery to a new method Done

fs/accounting/accounting.go

@@ -124,6 +124,7 @@ func (acc *Account) UpdateReader(in io.ReadCloser) {
 	acc.in = in
 	acc.close = in
 	acc.origIn = in
+	acc.closed = false
 	if acc.withBuf {
 		acc.WithBuffer()
 	}
@@ -243,14 +244,20 @@ func (acc *Account) Close() error {
 		return nil
 	}
 	acc.closed = true
-	close(acc.exit)
-	acc.stats.inProgress.clear(acc.name)
 	if acc.close == nil {
 		return nil
 	}
 	return acc.close.Close()
 }
 
+// Done with accounting - must be called to free accounting goroutine
+func (acc *Account) Done() {
+	acc.mu.Lock()
+	defer acc.mu.Unlock()
+	close(acc.exit)
+	acc.stats.inProgress.clear(acc.name)
+}
+
 // progress returns bytes read as well as the size.
 // Size can be <= 0 if the size is unknown.
 func (acc *Account) progress() (bytes, size int64) {