RDMA/hns: Optimize wqe buffer set flow for post send

Splits hns_roce_v2_post_send() into three sub-functions: set_rc_wqe(),
set_ud_wqe() and update_sq_db() to simplify the code.

Link: https://lore.kernel.org/r/1583839084-31579-6-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>