RDMA/hns: Package for hns_roce_rereg_user_mr function

Move some code of the hns_roce_rereg_user_mr() function into an
independent function in oder to improve readability.

Link: https://lore.kernel.org/r/1562593285-8037-8-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>