x/net/ipv6: fix using wrong constant in test
x/net/ipv6: fix using wrong constant in test

LGTM=iant
R=iant
CC=golang-codereviews
https://golang.org/cl/174480043

w4ngyi/net,mckinney-subgraph/net,yangxianjun/net,w4ngyi/net,raboof/net,DramaFever/golang.org-x-net,foreversmart/net,cesanta/goxnet,veliamsli/net,flowlo/go-net,zabawaba99/net,foreversmart/net,chenzlabs/net,subuk/net,cesanta/goxnet,dafyddcrosby/net,yangxianjun/net,albertjin/golang-net,kiip/net,mckinney-subgraph/net,subuk/net,cmars/tools,chenzlabs/net,zabawaba99/net,dafyddcrosby/net,raboof/net,veliamsli/net,gomini/net,DramaFever/golang.org-x-net,muzining/net,cmars/tools,albertjin/golang-net,muzining/net,golang/net,gomini/net,cmars/tools,kiip/net,cmars/tools,flowlo/go-net