RDMA/hns: Optimize mhop put flow for multi-hop addressing

Optimizes hns_roce_table_mhop_get() by encapsulating code about clearing
hem into clear_mhop_hem(), which will make the code flow clearer.

Link: https://lore.kernel.org/r/1584417324-2255-3-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>