vdpa/mlx5: Fix error return in map_direct_mr()

Fix to return the variable "err" from the error handling case instead
of "ret".

Fixes: 94abbccd ("vdpa/mlx5: Add shared memory registration code")
Signed-off-by: Jing Xiangfeng <jingxiangfeng@huawei.com>
Link: https://lore.kernel.org/r/20201026070637.164321-1-jingxiangfeng@huawei.com


Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Acked-by: Eli Cohen <elic@nvidia.com>
Cc: stable@vger.kernel.org
Acked-by: Jason Wang <jasowang@redhat.com>