lazy-pages: return to epoll_wait after completing forks

If we get fork() event just before transferring last IOV of the parent
process, continuing to background fetch after completing fork event
handling will cause lazy-pages daemon to exit and nothing will monitor the
child process memory.

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