BUG/MEDIUM: threads: fix a tiny race in thread_isolate()

BUG/MEDIUM: threads: fix a tiny race in thread_isolate()

Aurélien found a tiny race in thread_isolate() that can allow a thread
that was running under isolation to continue running while another one
enters isolation. The reason is that the check for harmless is only
done before winning the CAS, but since the previously isolated thread
doesn't wait for !rdv_request in thread_release(), it can effectively
continue its activities while the next one believes it's isolated. A
proper solution consists in looping once again in thread_isolate() to
recheck (and wait) for all threads to be isolated once the CAS is won.

The issue was introduced in 2.7 by commit 598cf3f22 ("MAJOR: threads:
change thread_isolate to support inter-group synchronization") so the
fix needs to be backported there.