fs/accounting: Fix "file already closed" on transfer retries

This was caused by the recent reworking of the accounting interface.
The Transfer object was recycling the Accounting object without
resetting the stream.

See: https://forum.rclone.org/t/error-file-already-closed/11469/
See: https://forum.rclone.org/t/rclone-b2-sync-post-error-method-not-supported/11718/

fs/accounting/transfer.go

@@ -117,6 +117,8 @@ func (tr *Transfer) Account(in io.ReadCloser) *Account {
 	tr.mu.Lock()
 	if tr.acc == nil {
 		tr.acc = newAccountSizeName(tr.stats, in, tr.size, tr.remote)
+	} else {
+		tr.acc.UpdateReader(in)
 	}
 	tr.mu.Unlock()
 	return tr.acc