mm: zswap: update comment for struct zswap_entry

Since commit 0bb48849 ("mm: zswap: remove zswap_header"), the 'offset'
has been replaced by swpentry, update the comment for it, and also add
comment for 'objcg'.

Link: https://lkml.kernel.org/r/20230808062056.292950-1-xiujianfeng@huaweicloud.com


Signed-off-by: Xiu Jianfeng <xiujianfeng@huawei.com>
Reviewed-by: Yosry Ahmed <yosryahmed@google.com>
Acked-by: Nhat Pham <nphamcs@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>