Only when calling the poll syscall the first time can user
receive POLLPRI correctly. After that, user always fails to
acquire the event signal.

Reproduce case:
1. Get the monitor code in Documentation/accounting/psi.txt
2. Run it, and wait for the event triggered.
3. Kill and restart the process.

The question is why we can end up with poll_scheduled = 1 but the work
not running (which would reset it to 0). And the answer is because the
scheduling side sees group->poll_kworker under RCU protection and then
schedules it, but here we cancel the work and destroy the worker. The
cancel needs to pair with resetting the poll_scheduled flag.
Signed-off-by: NJason Xing <kerneljasonxing@linux.alibaba.com>
Reviewed-by: NCaspar Zhang <caspar@linux.alibaba.com>
Reviewed-by: NSuren Baghdasaryan <surenb@google.com>
Acked-by: NJohannes Weiner <hannes@cmpxchg.org>
Signed-off-by: NJoseph Qi <joseph.qi@linux.alibaba.com>