If re-registering an MR in hns_roce_rereg_user_mr(), we should return NULL
instead of passing 0 to ERR_PTR for clarity.

Fixes: 4e9fc1da ("RDMA/hns: Optimize the MR registration process")
Link: https://lore.kernel.org/r/20210804125939.20516-1-yuehaibing@huawei.comSigned-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>