go.net/ipv4: fix bit clear
go.net/ipv4: fix bit clear

R=golang-dev, bradfitz
CC=golang-dev
https://codereview.appspot.com/9582043

gigq/go.net,ssor/go.net,go-mirrors/go.net,andrewzeneski/go.net,polydawn-ports/go.net,fernandez14/go.net