runtime: OS thread appears to be re-used despite never releasing runtime.LockOSThread() [1.11 backport]

[gopherbot]

@ianlancetaylor requested issue #28979 to be considered for backport to the next 1.11 minor release.

@gopherbot Please open a backport issue for 1.10 and 1.11.

[bcmills]

@mknyszek, @aclements, @ianlancetaylor: Now that the fix has landed on master, do we want to backport it?

[mknyszek]

Nearly forgot. On it.

[gopherbot]

Change https://golang.org/cl/155117 mentions this issue: [release-branch.go1.11] runtime: don't clear lockedExt on locked M when G exits

[gopherbot]

Closed by merging f500f13 to release-branch.go1.11.