epoll: Add a flag, EPOLLWAKEUP, to prevent suspend while epoll events are ready

epoll: Add a flag, EPOLLWAKEUP, to prevent suspend while epoll events are ready

When an epoll_event, that has the EPOLLWAKEUP flag set, is ready, a
wakeup_source will be active to prevent suspend. This can be used to
handle wakeup events from a driver that support poll, e.g. input, if
that driver wakes up the waitqueue passed to epoll before allowing
suspend.

The current implementation uses an extra wakeup_source when
ep_scan_ready_list runs. This can cause problems if a single thread
is polling on wakeup events and frequent non-wakeup events (events
usually arrive during thread freezing) using the same epoll file.

Signed-off-by: Arve Hjønnevåg <arve@android.com>
Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>