RDMA/hns: Optimize qp buffer allocation flow

Encapsulate qp buffer allocation related code into 3 functions:
alloc_qp_buf(), map_wqe_buf() and free_qp_buf().

Link: https://lore.kernel.org/r/1582526258-13825-5-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>