dbcheck: Allow a dangling forward link outside our known NCs

dbcheck: Allow a dangling forward link outside our known NCs

If we do not have the NC of the target object we can not be really sure
that the object is redundent and so we want to keep it for now
and not (as happened until now) break the dbcheck run made during the
replication stage of a "samba-tool domain backup rename".

BUG: https://bugzilla.samba.org/show_bug.cgi?id=14450

Signed-off-by: Andrew Bartlett <abartlet@samba.org>
Reviewed-by: Douglas Bagnall <douglas.bagnall@catalyst.net.nz>
(cherry picked from commit 05228c4e07013c0e6f78f1330b3b787271282ca8)

Autobuild-User(v4-11-test): Karolin Seeger <kseeger@samba.org>
Autobuild-Date(v4-11-test): Thu Aug  6 13:32:29 UTC 2020 on sn-devel-184