vfio/mlx5: fix error code in mlx5vf_precopy_ioctl()

The copy_to_user() function returns the number of bytes remaining to
be copied but we want to return a negative error code here.

Fixes: 0dce165b

 ("vfio/mlx5: Introduce vfio precopy ioctl implementation")
Signed-off-by: Dan Carpenter <error27@gmail.com>
Reviewed-by: Yishai Hadas <yishaih@nvidia.com>
Link: https://lore.kernel.org/r/Y5IKVknlf5Z5NPtU@kili
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>