linkbench: Synchronize more carefully for mtrace's sake
linkbench: Synchronize more carefully for mtrace's sake

aclements/sv6,aclements/sv6,bowlofstew/sv6,aclements/sv6,aclements/sv6,bowlofstew/sv6,aclements/sv6,bowlofstew/sv6,bowlofstew/sv6,bowlofstew/sv6