dbcheck: don't move already deleted objects to LostAndFound

dbcheck: don't move already deleted objects to LostAndFound

This would typically happen when the garbage collection
removed a parent object before a child object (both with
the DISALLOW_MOVE_ON_DELETE bit set in systemFlags),
while dbcheck is running at the same time as the garbage collection.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=13816

Signed-off-by: Stefan Metzmacher <metze@samba.org>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>
(cherry picked from commit 6d50ee74920c39cdb18b427bfaaf200775bf2d73)