runtime: fix TestLFStackStress
runtime: fix TestLFStackStress

Fixes issue 7138.

R=r, bradfitz, dave
CC=dvyukov, golang-codereviews
https://codereview.appspot.com/53910043

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