vdpa/mlx5: Set err = -ENOMEM in case dma_map_sg_attrs fails

Set err = -ENOMEM if dma_map_sg_attrs() fails so the function reutrns
error.

Fixes: 94abbccd

 ("vdpa/mlx5: Add shared memory registration code")
Signed-off-by: Eli Cohen <elic@nvidia.com>
Reported-by: kernel test robot <lkp@intel.com>
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Link: https://lore.kernel.org/r/20210411083646.910546-1-elic@nvidia.com
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Acked-by: Jason Wang <jasowang@redhat.com>
Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>