go.net/ipv4: make supportsNewIPInput const
go.net/ipv4: make supportsNewIPInput const

R=dave
CC=golang-dev
https://golang.org/cl/6822083

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