CVE-2020-25722 Ensure the structural objectclass cannot be changed

CVE-2020-25722 Ensure the structural objectclass cannot be changed

If the structural objectclass is allowed to change, then the restrictions
locking an object to remaining a user or computer will not be enforcable.

Likewise other LDAP inheritance rules, which allow only certain
child objects can be bypassed, which can in turn allow creation of
(unprivileged) users where only DNS objects were expected.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=14753
BUG: https://bugzilla.samba.org/show_bug.cgi?id=14889

Signed-off-by: Andrew Bartlett <abartlet@samba.org>
Reviewed-by: Joseph Sutton <josephsutton@catalyst.net.nz>