journal: when copying journal file to undo NOCOW flag, go via fd
    
    We have the journal file open already, hence reference it via the fd
    insted of the file name. After all, some other tool might have
    renamed/deleted it already.
    
    Let's not actually reuse the fd though, since we want a separate file
    offset for the copying, hence just make it simply and reopen via
    /proc/self/fd/.
    
    Follow-up for d71ece3f0b85c7a3decc50143b68ac07fc5831ae