runtime: fix a race in bgsweep See the comment for description.
runtime: fix a race in bgsweep
See the comment for description.

LGTM=rsc
R=rsc
CC=golang-codereviews
https://codereview.appspot.com/75670044

apparentlymart/golang,apparentlymart/golang,apparentlymart/golang,apparentlymart/golang,apparentlymart/golang,apparentlymart/golang,apparentlymart/golang