exfat: remove argument 'sector' from exfat_get_dentry()

No any function uses argument 'sector', remove it.

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