net: fix race in TestReadWriteDeadline.
net: fix race in TestReadWriteDeadline.

Discovered by adding OBLOCK support to race
instrumentation.

R=golang-dev, minux.ma, rsc
CC=golang-dev
http://codereview.appspot.com/6819067

wesolows/golang-old,wesolows/golang,wesolows/golang-old,wesolows/golang,firebitsbr/golang-old,wesolows/golang,wesolows/golang,wesolows/golang,firebitsbr/golang-old,firebitsbr/golang-old,firebitsbr/golang-old,wesolows/golang,firebitsbr/golang-old,firebitsbr/golang-old,wesolows/golang-old,firebitsbr/golang-old,wesolows/golang-old,wesolows/golang-old,wesolows/golang,wesolows/golang-old,firebitsbr/golang-old,wesolows/golang-old