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

rflanagan/reginaldflanagan-project1,bryanxu/go-zh,sanjosh/sanjos100-tipc,glycerine/jeaten-go-arrayof-structof,bryanxu/go-zh,rdp/rogerpack2005-golang,d0f/go-zh,mhennings/marcohennings-go,rdp/rogerpack2005-golang,mhennings/marcohennings-go,rflanagan/reginaldflanagan-project1,webfd/go-zh,rflanagan/reginaldflanagan-project1,mhennings/marcohennings-go,glycerine/jeaten-go-arrayof-structof,sanjosh/sanjos100-tipc,rdp/rogerpack2005-golang,scirelli/scirelli-go,sanjosh/sanjos100-tipc,scirelli/scirelli-go,rdp/rogerpack2005-golang,d0f/go-zh,scirelli/scirelli-go,webfd/go-zh,sanjosh/sanjos100-tipc,scirelli/scirelli-go,sanjosh/sanjos100-tipc,rdp/rogerpack2005-golang,mhennings/marcohennings-go,d0f/go-zh,d0f/go-zh,d0f/go-zh,webfd/go-zh,rflanagan/reginaldflanagan-project1,d0f/go-zh,webfd/go-zh,bryanxu/go-zh,scirelli/scirelli-go,sanjosh/sanjos100-tipc,glycerine/jeaten-go-arrayof-structof,d0f/go-zh,webfd/go-zh,glycerine/jeaten-go-arrayof-structof,webfd/go-zh,bryanxu/go-zh,bryanxu/go-zh,rflanagan/reginaldflanagan-project1,bryanxu/go-zh,mhennings/marcohennings-go,scirelli/scirelli-go,mhennings/marcohennings-go,scirelli/scirelli-go,rflanagan/reginaldflanagan-project1,mhennings/marcohennings-go,glycerine/jeaten-go-arrayof-structof,rdp/rogerpack2005-golang,scirelli/scirelli-go,glycerine/jeaten-go-arrayof-structof,glycerine/jeaten-go-arrayof-structof,d0f/go-zh,glycerine/jeaten-go-arrayof-structof,sanjosh/sanjos100-tipc,rflanagan/reginaldflanagan-project1,sanjosh/sanjos100-tipc,rdp/rogerpack2005-golang,webfd/go-zh,rdp/rogerpack2005-golang,webfd/go-zh,bryanxu/go-zh,mhennings/marcohennings-go,bryanxu/go-zh,rflanagan/reginaldflanagan-project1