[NETFILTER]: nf_conntrack: Fix missing check for ICMPv6 type

This makes nf_conntrack_icmpv6 check that ICMPv6 type isn't < 128
to avoid accessing out of array valid_new[] and invmap[].

Signed-off-by: Yasuyuki Kozakai <yasuyuki.kozakai@toshiba.co.jp>
Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>