net: fix intermittent TestAddFDReturnsError failure
net: fix intermittent TestAddFDReturnsError failure

A fix similar to CL 6859043 was effective in resolving the intermittent failure.

Fixes issue 4423.

R=golang-dev, rsc
CC=golang-dev
http://codereview.appspot.com/6854102

pombredanne/go-deleteme,pombredanne/go-deleteme,pombredanne/go-deleteme,pombredanne/go-deleteme,gnanderson/go,pombredanne/go-deleteme,pombredanne/go-deleteme,apparentlymart/golang,tv42/old-go,gnanderson/go,dsriram/golang,tv42/old-go,apparentlymart/golang,gnanderson/go,gnanderson/go,dsriram/golang,dsriram/golang,pombredanne/go-deleteme,gnanderson/go,tv42/old-go,apparentlymart/golang,dsriram/golang,apparentlymart/golang,gnanderson/go,tv42/old-go,apparentlymart/golang,tv42/old-go,tv42/old-go,dsriram/golang,apparentlymart/golang,apparentlymart/golang,dsriram/golang,gnanderson/go,tv42/old-go,dsriram/golang