lazy-pages: reset poll_timeout to 0 as soon as restore is finished

It is possible that notification about restore finish arrives at the same
time with a fork event. In such case we return to epoll_run_rfds without
resetting the poll_timeout and then we'll keep polling for events
indefinitely. To avoid this, we reset the poll_timeout to 0 as soon as we
know that restore is finished.

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