dsdb: Allow delete (directly and over DRS) of an object with a link to itself

dsdb: Allow delete (directly and over DRS) of an object with a link to itself

Previously this would fail with Unsupported critical extension 1.3.6.1.4.1.7165.4.3.2

Reported by Alexander Harm.  Many thanks for helping make Samba better
and for your patience with patches and providing debugging information.

REF: https://lists.samba.org/archive/samba/2020-February/228153.html
BUG: https://bugzilla.samba.org/show_bug.cgi?id=14306

Signed-off-by: Andrew Bartlett <abartlet@samba.org>
Reviewed-by: Andreas Schneider <asn@samba.org>