lazy-pages: don't close page_read when a parent process exits

If there were fork()'s during lazy-restore we create a shallow copy of the
parent process' page-read. Since all the copies reference the same open
files, we cannot close the page-read until we finish restore of all the
processes that share the page-read.

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