After

    # mount --rbind /tmp /mnt

(on 2.5.68) I have a corrupted namespace. Umounting /mnt fails,
and /proc/mounts contains

    ...
    /dev/root /mnt ext3 rw 0 0
    proc /mnt/proc proc rw 0 0
    usbfs /mnt/proc/bus/usb usbfs rw 0 0
    /dev/hdb5 /mnt/usr reiserfs rw 0 0
    ...

where of course no directories /mnt/proc or /mnt/usr exist.

This is caused by the fact that copy_tree() thinks that the dentry
it is called with is the root of the filesystem. If it is not,
confusion arose.

This patch fixes this confusion.