vfio: fix a missed vfio group put in vfio_pin_pages

When error occurs, need to put vfio group after a successful get.

Fixes: 95fc87b4 ("vfio: Selective dirty page tracking if IOMMU backed device pins pages")
Signed-off-by: Yan Zhao <yan.y.zhao@intel.com>
Reviewed-by: Cornelia Huck <cohuck@redhat.com>
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>