ipa-sam: use smbldap_set_bind_callback for Samba 4.7 or later
Samba 4.7 tightens up smbldap API by making 'struct smbldap_state' an
opaque. This means ipa-sam module cannot anymore directly set its
LDAP bind callback.
Use new smbldap API to set the LDAP bind callback.
Fixes https://pagure.io/freeipa/issue/6877
Reviewed-By: Martin Basti <mbasti@redhat.com>