CVE-2016-2110: auth/ntlmssp: don't let ntlmssp_handle_neg_flags() change ntlmssp_state->use_ntlmv2

ntlmssp_handle_neg_flags() can only disable flags, but not
set them. All supported flags are set at start time.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=11644

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