cifs: reinstate sec=ntlmv2 mount option

sec=ntlmv2 as a mount option got dropped in the mount option overhaul.

Cc: Sachin Prabhu <sprabhu@redhat.com>
Cc: <stable@vger.kernel.org> # 3.4+
Reported-by: Günter Kukkukk <linux@kukkukk.com>
Signed-off-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Steve French <smfrench@gmail.com>