RDMA/hns: Correct error in calculation of HNS_ROCE_HEM_CHUNK_LEN

driver inclusion
category: bugfix
bugzilla: NA
CVE: NA

We should consider size of pointer pointing to buf field in struct
hns_roce_hem_chunk when calculating HNS_ROCE_HEM_CHUNK_LEN, or its value
may be larger than we expected.

Feature or Bugfix: Bugfix

Signed-off-by: Sirong Wang <wangsirong@huawei.com>
Signed-off-by: Weihang Li <liweihang@hisilicon.com>
Reviewed-by: chenglang <chenglang@huawei.com>
Reviewed-by: Yixian Liu <liuyixian@huawei.com>
Reviewed-by: Yang Yingliang <yangyingliang@huawei.com>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>