exfat: remove call ilog2() from exfat_readdir()

There is no need to call ilog2() for the conversions between
cluster and dentry in exfat_readdir(), because these conversions
can be replaced with EXFAT_DEN_TO_CLU()/EXFAT_CLU_TO_DEN().

Code refinement, no functional changes.

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>