netfilter: x_tables: allow to use default cgroup match

There's actually no good reason why we cannot use cgroup id 0,
so lets just remove this artificial barrier.

Reported-by: Alexey Perevalov <a.perevalov@samsung.com>
Signed-off-by: Daniel Borkmann <dborkman@redhat.com>
Tested-by: Alexey Perevalov <a.perevalov@samsung.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>