vfio: Fix handling of error returned by 'vfio_group_get_from_dev()'

'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: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>