dsdb-acl: use dsdb_get_structural_oc_from_msg() rather than class_schemaid_guid_by_lD...

dsdb-acl: use dsdb_get_structural_oc_from_msg() rather than class_schemaid_guid_by_lDAPDisplayName

This uses dsdb_get_last_structural_objectclass(), which encodes this ordering
knowledge in one place in the code, rather than using this uncommented
magic expression:

(char *)oc_el->values[oc_el->num_values-1].data

Andrew Bartlett

Reviewed-by: Stefan Metzmacher <metze@samba.org>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>
(cherry picked from commit 6ab41506857814d69d897471a14002d98fb4c172)