cifs: add new "Unspecified" securityEnum value

Add a new securityEnum value to cover the case where a sec= option
was not explicitly set.

Signed-off-by: Jeff Layton <jlayton@redhat.com>
Reviewed-by: Pavel Shilovsky <piastry@etersoft.ru>
Signed-off-by: Steve French <smfrench@gmail.com>