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

Splits hns_roce_table_mhop_get() into 4 sub-functions to make the code flow
clearer.

Link: https://lore.kernel.org/r/1584417324-2255-2-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>