This is already pushed in my oom killer rewrite as patch 13/18 "oom: avoid
race for oom killed tasks detaching mm prior to exit".
It's not vital to merge now because causing the oom killer to temporarily
become a no-op before it can fully exit even though it has already
detached its memory only delays killing another task until it exits and
there's nothing in the way of that exiting while it's still under