copy_tree(): don't set ->mnt_mountpoint on the root of copy

copy_tree(): don't set ->mnt_mountpoint on the root of copy

It never made any sense - neither when copy_tree() had been introduced
(2.4.11-pre5), nor at any point afterwards.  Mountpoint is meaningless
without parent mount and the root of copied tree has no parent until we get
around to attaching it somewhere.  At that time we'll have mountpoint set;
before that we have no idea which dentry will be used as mountpoint.
IOW, copy_tree() should just leave the default value.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>