RDMA/hns: Optimize wqe buffer filling process for post send

Encapsulates the wqe buffer process details for datagram seg, fast mr seg
and atomic seg.

Link: https://lore.kernel.org/r/1583839084-31579-3-git-send-email-liweihang@huawei.com


Signed-off-by: Xi Wang <wangxi11@huawei.com>
Signed-off-by: Weihang Li <liweihang@huawei.com>
Reviewed-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>