CVE-2016-2111: s3:rpc_server/netlogon: always go through netr_creds_server_step_check()

The ensures we apply the "server schannel = yes" restrictions.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=11749

Pair-Programmed-With: Stefan Metzmacher <metze@samba.org>

Signed-off-by: Guenther Deschner <gd@samba.org>
Signed-off-by: Stefan Metzmacher <metze@samba.org>
