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

drivers/infiniband/hw/hns/hns_roce_device.h

@@ -673,7 +673,6 @@ struct hns_roce_qp {
 	/* this define must less than HNS_ROCE_MAX_BT_REGION */
 #define HNS_ROCE_WQE_REGION_MAX	 3
 	struct hns_roce_buf_region regions[HNS_ROCE_WQE_REGION_MAX];
-	int			region_cnt;
 	int                     wqe_bt_pg_shift;
 
 	u32			buff_size;