exfat: introduce exfat_chain_advance helper

exfat: introduce exfat_chain_advance helper

Introduce exfat_chain_advance() to walk a exfat_chain structure by a
given step, updating both ->dir and ->size fields atomically. This
helper handles both ALLOC_NO_FAT_CHAIN and ALLOC_FAT_CHAIN modes with
proper boundary checking.

Suggested-by: Yuezhang Mo <Yuezhang.Mo@sony.com>
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>