exfat: support multi-cluster for exfat_get_cluster

exfat: support multi-cluster for exfat_get_cluster

This patch introduces a count parameter to exfat_get_cluster, which
serves as an input parameter for the caller to specify the desired
number of clusters, and as an output parameter to store the length
of consecutive clusters.

This patch can improve read performance by reducing the number of
get_block calls in sequential read scenarios. speacially in small
cluster size.

According to my test data, the performance improvement is
approximately 10% when read FAT_CHAIN file with 512 bytes of
cluster size.

454 MB/s -> 511 MB/s

Suggested-by: Yuezhang Mo <Yuezhang.Mo@sony.com>
Signed-off-by: Chi Zhiling <chizhiling@kylinos.cn>
Reviewed-by: Yuezhang Mo <Yuezhang.Mo@sony.com>
Signed-off-by: Namjae Jeon <linkinjeon@kernel.org>