accounting: reset bytes read during copy retry - fixes #4178

During a copy/sync command, if an operation fails due to a network
issue and is retried, the underlying io.Reader is re-initialised,
but the stats for bytes already read are not reset, leading to incorrect
stats. THis was fixed by resetting the bytes read when an Account is
re-initialized.

fs/accounting/accounting.go

@@ -138,6 +138,8 @@ func (acc *Account) UpdateReader(in io.ReadCloser) {
 	acc.close = in
 	acc.origIn = in
 	acc.closed = false
+	acc.lpBytes = 0
+	acc.bytes = int64(0)
 	if withBuf {
 		acc.WithBuffer()
 	}