netfilter: conntrack: do not print icmpv6 as unknown via /proc

commit fbea31808ca124dd73ff6bb1e67c9af4607c3e32 upstream.

/proc/net/nf_conntrack shows icmpv6 as unknown.

Fixes: 09ec82f5 ("netfilter: conntrack: remove protocol name from l4proto struct")
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>