exfat: fix the newly allocated clusters are not freed in error handling

In error handling 'free_cluster', before num_alloc clusters allocated,
p_chain->size will not updated and always 0, thus the newly allocated
clusters are not freed.

Signed-off-by: Yuezhang Mo <Yuezhang.Mo@sony.com>
Reviewed-by: Andy Wu <Andy.Wu@sony.com>
Reviewed-by: Sungjong Seo <sj1557.seo@samsung.com>
Signed-off-by: Namjae Jeon <linkinjeon@kernel.org>