exfat: code cleanup for exfat_readdir()

For the root directory and other directories, the clusters
allocated to them can be obtained from exfat_inode_info, and
there is no need to distinguish them.

And there is no need to initialize atime/ctime/mtime/size in
exfat_readdir(), because exfat_iterate() does not use them.

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