timer: set O_NONBLOCK on the timerfd

timer: set O_NONBLOCK on the timerfd

Resetting a timerfd empties the data on the fd, so if the timer is reset
between triggering and us reading it, we may block trying to read it.

Since we read events off a device in a loop, a device sending a continuous
flow of events may cause the timer to trigger but delay reading it. If one of
the events cause e.g. the tap timer to be set, the timerfd may be empty at the
time of reading.

Suggested-by: Derek Foreman <derekf@osg.samsung.com>
Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>