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

stable inclusion
from linux-4.19.189
commit cb87700ccb0d4cbfc430d42bcf07029a63943d3a

--------------------------------

commit fbea3180 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>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>