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

tv42/old-go,tv42/old-go,pombredanne/go-deleteme,tv42/old-go,tv42/old-go,tv42/old-go,pombredanne/go-deleteme,pombredanne/go-deleteme,pombredanne/go-deleteme,pombredanne/go-deleteme,pombredanne/go-deleteme,tv42/old-go,tv42/old-go,pombredanne/go-deleteme