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>