tmpfs: don't try to dump mountpoint at 0 fd

If we happen to get 0 as the fd for the mountpoint, there is an error:

(00.280617) Dumping task cwd id 0x2b root id 0x2b
(00.280697) mnt: Dumping mountpoints
(00.280702) mnt:        132: 34:/ @ ./run/lock
(00.280714) mnt: Path `/run/lock' resolved to `./run/lock' mountpoint
tar: /proc/self/fd/0: Cannot open: Not a directory
tar: Error is not recoverable: exiting now
(00.283581) Error (util.c:666): exited, status=2
(00.283598) Error (mount.c:1220): mnt: Can't dump tmpfs content

Instead, let's not ever use 0 as the mountpoint fd.
Signed-off-by: Tycho Andersen <tycho.andersen@canonical.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>

mount.c

@@ -1190,6 +1190,13 @@ static int tmpfs_dump(struct mount_info *pm)
 	if (fd < 0)
 		return -1;
 
+	/* if fd happens to be 0 here, we need to move it to something
+	 * non-zero, because cr_system_userns closes STDIN_FILENO as we are not
+	 * interested in passing stdin to tar.
+	 */
+	if (move_img_fd(&fd, STDIN_FILENO) < 0)
+		goto out;
+
 	if (fcntl(fd, F_SETFD, fcntl(fd, F_GETFD) & ~FD_CLOEXEC) == -1) {
 		pr_perror("Can not drop FD_CLOEXEC");
 		goto out;