exfat: do not sync parent dir if just update timestamp

When sync or dir_sync is enabled, there is no need to sync the
parent directory's inode if only for updating its timestamp.

1. If an unexpected power failure occurs, the timestamp of the
   parent directory is not updated to the storage, which has no
   impact on the user.

2. The number of writes will be greatly reduced, which can not
   only improve performance, but also prolong device life.

Signed-off-by: Yuezhang Mo <Yuezhang.Mo@sony.com>
Reviewed-by: Andy Wu <Andy.Wu@sony.com>
Reviewed-by: Aoyama Wataru <wataru.aoyama@sony.com>
Reviewed-by: Sungjong Seo <sj1557.seo@samsung.com>
Signed-off-by: Namjae Jeon <linkinjeon@kernel.org>