[release-branch.go1.1] net: fix dial race on plan9 and windows

««« CL 9159043 / f1ddc3ce3dfe
net: fix dial race on plan9 and windows

Fixes #5349.

R=golang-dev, lucio.dere, dsymonds, bradfitz, iant, adg, dave, r
CC=golang-dev
https://golang.org/cl/9159043
»»»

R=golang-dev
CC=golang-dev
https://golang.org/cl/9315043