RDMA/hns: Optimize hns_roce_mhop_alloc function.

Move some lines for allocating multi-hop addressing into independent
functions in order to improve readability.

Link: https://lore.kernel.org/r/1562593285-8037-7-git-send-email-oulijun@huawei.com


Signed-off-by: Lang Cheng <chenglang@huawei.com>
Signed-off-by: Lijun Ou <oulijun@huawei.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>