net: do not start io if deadline has passed
net: do not start io if deadline has passed

R=golang-dev, mikioh.mikioh, minux.ma, bradfitz
CC=golang-dev
http://codereview.appspot.com/6851098

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