vfio: Rework group release notifier warning

vfio: Rework group release notifier warning

The intent of the original warning is make sure that the mdev vendor
driver has removed any group notifiers at the point where the group
is closed by the user.  Theoretically this would be through an
orderly shutdown where any devices are release prior to the group
release.  We can't always count on an orderly shutdown, the user can
close the group before the notifier can be removed or the user task
might be killed.  We'd like to add this sanity test when the group is
idle and the only references are from the devices within the group
themselves, but we don't have a good way to do that.  Instead check
both when the group itself is removed and when the group is opened.
A bit later than we'd prefer, but better than the current over
aggressive approach.

Fixes: 2607b5287206 ("vfio: support notifier chain in vfio_group")
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>
Cc: <stable@vger.kernel.org> # v4.10
Cc: Jike Song <jike.song@intel.com>