harness: Add fallback code for filesystems not supporting O_DIRECT
    
    When running the harness on a filesystem such as a tmpfs, which do not
    support O_DIRECT, fallback to calls without the flag.
    
    Signed-off-by: Guillem Jover <guillem@hadrons.org>
    [JEM: change from duplicating the open call to using F_SETFL]
    [JEM: 18 and 21 require O_DIRECT-skip if not present]
    Signed-off-by: Jeff Moyer <jmoyer@redhat.com>