cifs: reinstate sec=ntlmv2 mount option

commit 7659624ffb550d69c87f9af9ae63e717daa874bd upstream.

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

Cc: Sachin Prabhu <sprabhu@redhat.com>
Reported-by: Günter Kukkukk <linux@kukkukk.com>
Signed-off-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Steve French <smfrench@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>