mount: build tree of mounts before dumping them

When CRIU wants to dump content, it checks that nothing is overmounted.
The list of children for such mounts must be empty, but these lists are
filled during constructing a tree of mounts.
Signed-off-by: Andrey Vagin <avagin@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>

mount.c

@@ -469,6 +469,9 @@ int dump_mnt_ns(int ns_pid, struct cr_fdset *fdset)
 		return -1;
 	}
 
+	if (mnt_build_tree(pm) == NULL)
+		return -1;
+
 	if (validate_shared(mntinfo)) {
 		pr_err("Can't proceed %d's mountinfo\n", ns_pid);
 		return -1;