RDMA/hns: Optimize qp doorbell allocation flow

Encapsulate the kernel qp doorbell allocation related code into 2
functions: alloc_qp_db() and free_qp_db().

Link: https://lore.kernel.org/r/1582526258-13825-8-git-send-email-liweihang@huawei.com


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