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

h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang