go.net/ipv6: remove unnecessary bit masking
go.net/ipv6: remove unnecessary bit masking

R=dave
CC=golang-dev
https://golang.org/cl/11666045

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