mounts: Connect sub-root to the main root as children

Each sub-namespace is restored as sub-tree of the root mntns, so
the parent of sub-mntns root is the root of the root mntns.
Signed-off-by: Andrey Vagin <avagin@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>

mount.c

@@ -232,6 +232,15 @@ static struct mount_info *mnt_build_ids_tree(struct mount_info *list)
 			pr_err("Mountpoint %d w/o parent %d found @%s (root %s)\n",
 					m->mnt_id, m->parent_mnt_id, m->mountpoint,
 					root ? "found" : "not found");
+			if (root && m->is_ns_root) {
+				/*
+				 * A root of a sub mount namespace is
+				 * mounted in a temporary directory in the
+				 * root mount namespace, so its parent is
+				 * the main root.
+				 */
+				p = root;
+			} else
 				return NULL;
 		}