exfat: fix overflow for large capacity partition

Using int type for sector index, there will be overflow in a large
capacity partition.

For example, if storage with sector size of 512 bytes and partition
capacity is larger than 2TB, there will be overflow.

Fixes: 1b6138385499 ("exfat: reduce block requests when zeroing a cluster")

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>
Acked-by: Sungjong Seo <sj1557.seo@samsung.com>
Signed-off-by: Namjae Jeon <linkinjeon@kernel.org>