network: update comment and log message
    
    After 4b30f2e135ee84041bb597edca7225858f4ef4fb, reading stable_secret
    sysctl property fails with -ENOMEM, instead of -EIO.
    This is due to read_full_virtual_file() uses read() as the backend while
    read_one_line_file() uses fgetc(). And each functions return different
    error on fails.
    
    Anyway, the failure is harmless here. So, the log message and comment is
    updated.
    
    Closes one of the issues in #19410.
    
    (cherry picked from commit 0e27527170aa0179f2a4b3b55b78ddc049f815b9)