lazy-pages: make complete_forks more robust

The complete_forks function presumes that it always has a work to do
because we assume that fork event is the only case when we drop out of
epoll_run_rfds with positive return value.

Teach complete_forks to bail out when there is no pending forks to process
to allow exiting epoll_run_rfds for different reasons.

Signed-off-by: Mike Rapoport <rppt@linux.vnet.ibm.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>