RDMA/hns: Fix return in hns_roce_rereg_user_mr()

RDMA/hns: Fix return in hns_roce_rereg_user_mr()

[ Upstream commit c4c7d7a43246a42b0355692c3ed53dff7cbb29bb ]

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: 4e9fc1dae2a9 ("RDMA/hns: Optimize the MR registration process")
Link: https://lore.kernel.org/r/20210804125939.20516-1-yuehaibing@huawei.com
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>