netfilter: cthelper: add missing attribute validation for cthelper

commit c049b3450072b8e3998053490e025839fecfef31 upstream.

Add missing attribute validation for cthelper
to the netlink policy.

Fixes: 12f7a505 ("netfilter: add user-space connection tracking helper infrastructure")
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>