message: [firewall] remove `-p ip` argument in ip6tables rules
  Removing the `-p ip` argument because it's a no-op and it may not be
  supported by some old versions of iptables.
diff: @@ -64,11 +64,11 @@ firewall_start()
  ip6tables -N $OTBR_FORWARD_INGRESS_CHAIN
  ip6tables -I FORWARD 1 -o $THREAD_IF -j $OTBR_FORWARD_INGRESS_CHAIN
  - ip6tables -A $OTBR_FORWARD_INGRESS_CHAIN -m pkttype --pkt-type unicast -i $THREAD_IF -p ip -j DROP
  - ip6tables -A $OTBR_FORWARD_INGRESS_CHAIN -m set --match-set otbr-ingress-deny-src src -p ip -j DROP
  - ip6tables -A $OTBR_FORWARD_INGRESS_CHAIN -m set --match-set otbr-ingress-allow-dst dst -p ip -j ACCEPT
  - ip6tables -A $OTBR_FORWARD_INGRESS_CHAIN -m pkttype --pkt-type unicast -p ip -j DROP
  - ip6tables -A $OTBR_FORWARD_INGRESS_CHAIN -p ip -j ACCEPT
  + ip6tables -A $OTBR_FORWARD_INGRESS_CHAIN -m pkttype --pkt-type unicast -i $THREAD_IF -j DROP
  + ip6tables -A $OTBR_FORWARD_INGRESS_CHAIN -m set --match-set otbr-ingress-deny-src src -j DROP
  + ip6tables -A $OTBR_FORWARD_INGRESS_CHAIN -m set --match-set otbr-ingress-allow-dst dst -j ACCEPT
  + ip6tables -A $OTBR_FORWARD_INGRESS_CHAIN -m pkttype --pkt-type unicast -j DROP
  + ip6tables -A $OTBR_FORWARD_INGRESS_CHAIN -j ACCEPT
  }
  firewall_stop()
  