RDMA/hns: Correct the value of HNS_ROCE_HEM_CHUNK_LEN

Size of pointer to buf field of struct hns_roce_hem_chunk should be
considered when calculating HNS_ROCE_HEM_CHUNK_LEN, or sg table size will
be larger than expected when allocating hem.

Fixes: 9a443537 ("IB/hns: Add driver files for hns RoCE driver")
Link: https://lore.kernel.org/r/1572575610-52530-2-git-send-email-liweihang@hisilicon.com


Signed-off-by: Sirong Wang <wangsirong@huawei.com>
Signed-off-by: Weihang Li <liweihang@hisilicon.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>