CVE-2016-2111: auth/gensec: require DCERPC_AUTH_LEVEL_INTEGRITY or higher in schannel_update()

It doesn't make any sense to allow other auth levels.

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

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