lazy-pages: don't close page-read for child processes

The page-read for child process is a shallow copy of the parent process
page-read. They share the open file descriptors and the pagemap.
The lpi_fini of the child processes should not release any resources, they
all will be released during lpi_fini of the parent process.

Fixes: #325

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