vfio/mlx5: error pointer dereference in error handling

This code frees the wrong "buf" variable and results in an error pointer
dereference.

Fixes: 34e2f271

 ("vfio/mlx5: Introduce multiple loads")
Signed-off-by: Dan Carpenter <error27@gmail.com>
Reviewed-by: Yishai Hadas <yishaih@nvidia.com>
Link: https://lore.kernel.org/r/Y5IKia5SaiVxYmG5@kili
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>