runtime: if span was scavenged, mark it as needing to be zeroed
runtime: if span was scavenged, mark it as needing to be zeroed

Update issue 4979.

R=dvyukov, r, bradfitz
CC=golang-dev
https://codereview.appspot.com/8697045

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