librbd: do not invalidate object map if update races with copyup

librbd: do not invalidate object map if update races with copyup

The copyup state machine needs to iterate over all object maps to update
the existence for the object. If an snapshot is being removed concurrently,
it's possible to invalidate the object map for the image.

Fixes: http://tracker.ceph.com/issues/24516
Signed-off-by: Jason Dillaman <dillaman@redhat.com>