exfat: introduce exfat_cluster_walk helper

exfat: introduce exfat_cluster_walk helper

Introduce exfat_cluster_walk() to walk the FAT chain by a given step,
handling both ALLOC_NO_FAT_CHAIN and ALLOC_FAT_CHAIN modes. Also
redefine exfat_get_next_cluster as a thin wrapper around it for
backward compatibility.

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