fs/cifs: correctly to anonymous authentication for the NTLM(v2) authentication

Only server which map unknown users to guest will allow
access using a non-null NTLMv2_Response.

For Samba it's the "map to guest = bad user" option.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=11913



Signed-off-by: Stefan Metzmacher <metze@samba.org>
CC: Stable <stable@vger.kernel.org>
Signed-off-by: Steve French <smfrench@gmail.com>