net/sched: cls_flower: Correct matching on ICMPv6 code

When matching on the ICMPv6 code ICMPV6_CODE rather than
ICMPV4_CODE attributes should be used.

This corrects what appears to be a typo.

Sample usage:

tc qdisc add dev eth0 ingress
tc filter add dev eth0 protocol ipv6 parent ffff: flower \
	indev eth0 ip_proto icmpv6 type 128 code 0 action drop

Without this change the code parameter above is effectively ignored.

Fixes: 7b684884 ("net/sched: cls_flower: Support matching on ICMP type and code")
Signed-off-by: Simon Horman <simon.horman@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>