'vfio_group_get_from_dev()' seems to return only NULL on error, not an
error pointer.

Fixes: 2169037d ("vfio iommu: Added pin and unpin callback functions to vfio_iommu_driver_ops")
Fixes: c086de81 ("vfio iommu: Add blocking notifier to notify DMA_UNMAP")
Signed-off-by: NChristophe JAILLET <christophe.jaillet@wanadoo.fr>
Reported-by: NDan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: NAlex Williamson <alex.williamson@redhat.com>