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>

page-read.c

@@ -217,6 +217,7 @@ err_cl:
 int open_page_read_at(int dfd, int pid, struct page_read *pr, int flags, bool shmem)
 {
 	pr->pe = NULL;
+	pr->parent = NULL;
 	pr->bunch.iov_len = 0;
 	pr->bunch.iov_base = NULL;