runtime: debug.FreeOSMemory not freeing memory

[mknyszek]

At tip, debug.FreeOSMemory isn't doing what its documentation is saying it will do. Notably, HeapReleased doesn't increase all the way to HeapIdle. This failure is obvious in https://github.com/savalin/example/blob/master/main.go and this was discovered in investigating #35848.

This is happening because the changes to debug.FreeOSMemory in the page allocator don't do the right thing. The fix is to reset the scavenger address, then scavenge.

This may need to block the beta, but the fix is known and easy. I will put up a change ASAP.

CC @aclements @andybons

[mknyszek]

I've uploaded a fix which I've confirmed resolves the issue.

[gopherbot]

Change https://golang.org/cl/208960 mentions this issue: runtime: reset scavenge address in scavengeAll