conntrack: CONNLABELS are not a nested attribute

This fixes construction of the conntrack object when CTA_LABEL
attribute is present.

Signed-off-by: Florian Westphal <fw@strlen.de>
conntrack: connlabel: remove useless test

Can't be zero, it was already tested.

Signed-off-by: Florian Westphal <fw@strlen.de>