[dev.garbage] runtime: simplifiy lfstack.c due to undiagnosed buffer corruption.
The changes got rid of the problems we were seeing.
We suspect the pushcnt field has a race.

LGTM=rsc
R=dvyukov, rsc
CC=golang-codereviews
https://codereview.appspot.com/159330043

Committer: Russ Cox <rsc@golang.org>