exfat: fix missing REQ_SYNC in exfat_update_bhs()

If 'dirsync' is enabled, all directory updates within the
filesystem should be done synchronously. exfat_update_bh()
does as this, but exfat_update_bhs() does not.

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