exfat: fix overflow in sector and cluster conversion

According to the exFAT specification, there are at most 2^32-11
clusters in a volume. so using 'int' is not enough for cluster
index, the return value type of exfat_sector_to_cluster() should
be 'unsigned int'.

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