netfilter: nf_conntrack_ipv6: fix comment for packets without data

Remove ambiguity of double negation.

Signed-off-by: Florent Fourcot <florent.fourcot@enst-bretagne.fr>
Acked-by: Rick Jones <rick.jones2@hp.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>