INI: Return errno, not -1 on failure from sss_ini_get_stat
    
    sss_ini_get_stat() has two branches for two libini versions. The newer
    version directly returns EIO on failure, but the old version would have
    returned the return value from fstat() directly. And fstat() returns -1
    on failure but sets errno. This patch returns errno on failure and EOK
    on success.
    
    Reviewed-by: Michal Židek <mzidek@redhat.com>