lazy-pages: handle_requests: fix return value propagation

When we return from epoll_run_rfds with positive return value it means that
event handling loop was interrupted because the event should be handled
outside of that loop. Is always the case with UFFD_EVENT_FORK.

It may happen that the event occurred after we've completed the memory
transfer and we are on the way to successful return from the
handle_requests() function, but instead of returning 0 we will return the
positive value we've got from epoll_run_rfds.

Explicitly assigning return value of complete_forks() fixes this issue.

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