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>

criu/uffd.c

@@ -965,6 +965,9 @@ static int complete_forks(int epollfd, struct epoll_event **events, int *nr_fds)
 {
 	struct lazy_pages_info *lpi, *n;
 
+	if (list_empty(&pending_lpis))
+		return 0;
+
 	list_for_each_entry(lpi, &pending_lpis, l)
 		(*nr_fds)++;