Remove flush(stdout) in pkg/chrootarchive/diff_unix.go and improve error reporting of flush() to fix #21103

pkg/chrootarchive/diff_unix.go erroneously calls flush on stdout, which tries to read from stdout returning an error.
This has been fixed by removing the call and by modifying flush to return errors and checking for these errors on calls to flush.

Signed-off-by: Amit Krishnan <krish.amit@gmail.com>