exfat: tweak cluster cache to support zero offset

exfat: tweak cluster cache to support zero offset

The current cache mechanism does not support reading clusters starting
from a file offset of zero. This patch enables that feature in
preparation for subsequent reads of contiguous clusters from offset zero.

1. support finding clusters with zero offset.
2. allow clusters with zero offset to be cached.

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