timerfd: Kill post_open stage

Timerfd's post_open state does not depend on another objects,
so it may be safely merged into open stage.

travis-ci: success for Rework file opening scheme to make it asynchronous (rev5)
Signed-off-by: Kirill Tkhai <ktkhai@virtuozzo.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>