page-read: initialize parent to NULL

For example restore_shmem_content allocates the page_read structure on
stack.

Cc: Pavel Emelyanov <xemul@parallels.com>
Reported-by: Jenkins Criuovich
Signed-off-by: Andrey Vagin <avagin@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>