exfat: use exfat_chain_advance helper

exfat: use exfat_chain_advance helper

Replace open-coded cluster chain walking logic with exfat_chain_advance()
across exfat_readdir, exfat_find_dir_entry, exfat_count_dir_entries,
exfat_search_empty_slot and exfat_check_dir_empty.

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>