librbd: ensure image cannot be closed until in-flight IO callbacks complete

librbd: ensure image cannot be closed until in-flight IO callbacks complete

If a librbd client attempts to close the image while it still has in-flight IO
pending, it's possible for the AsyncOperation tracker which prevents the image
from being closed to be completed before the actual AioCompletion callback
fires. This can result in the now destructed ImageCtx being de-referenced by
the AioCompletion.

Fixes: https://tracker.ceph.com/issues/46737
Signed-off-by: Jason Dillaman <dillaman@redhat.com>