mount: clone a mount namespace to open a detached mount

If we can't create a temporary directory for a detached mount,
we can clone a whole mount namespace, open a mount and release
the created namespace. The result will be the same.

https://jira.sw.ru/browse/PSBM-57135
https://github.com/opencontainers/runc/issues/1202
travis-ci: success for A few fixes to c/r a docker container with a console (rev3)
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>

criu/mount.c

@@ -1091,9 +1091,22 @@ int open_mountpoint(struct mount_info *pm)
 		goto out;
 
 	mnt_path = get_clean_mnt(pm, mnt_path_tmp, mnt_path_root);
-	if (mnt_path == NULL)
+	if (mnt_path == NULL) {
+		/*
+		 * We probably can't create a temporary direcotry,
+		 * so we can try to clone the mount namespace, open
+		 * the required mount and destroy this mount namespace
+		 * by calling restore_ns() below in this function.
+		 */
+		if (unshare(CLONE_NEWNS)) {
+			pr_perror("Unable to clone a mount namespace");
 			goto out;
+		}
 
+		fd = open(pm->mountpoint, O_RDONLY | O_DIRECTORY, 0);
+		if (fd < 0)
+			pr_perror("Can't open directory %s: %d", pm->mountpoint, fd);
+	} else
 		fd = open_detach_mount(mnt_path);
 	if (fd < 0)
 		goto out;