RDMA/hns: RDMA/hns: Assign rq head pointer when enable rq record db

When flush cqe, it needs to get the pointer of rq and sq from db address
space of user and update it into qp context by modified qp. if rq does not
exist, it will not get the value from db address space of user.

Signed-off-by: Lijun Ou <oulijun@huawei.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>