RDMA/hns: Assign dest_qp when deregistering mr

It needs to create eight reserve QPs for resolving
a bug of hip06. When deregistering mr, it will issue
a rdma write for every reserve QPs.

When modify qp from init to rtr, it needs to set
the value of dest_qp_num. Otherwise, it will lead
an error of freeing mr.

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