RFR: Concurrent partial should fix up roots only on cancel

[Aleksey Shipilev]

On 10/10/2017 03:45 PM, Roman Kennke wrote:
> We don't need to fix anything if there was no work to to, right?

Yes, we need to fix stuff only on cancellation path. The whole thing about has_work needs to be
refactored out: there is no need to even get into final-partial if _has_work = false. Current patch
fixes the test failure Zhengyu has on aarch64.

Thanks,
-Aleksey