attach_recursive_mnt(): do not lock the covering tree when sliding something under it

If we are propagating across the userns boundary, we need to lock the
mounts added there.  However, in case when something has already
been mounted there and we end up sliding a new tree under that,
the stuff that had been there before should not get locked.

IOW, lock_mnt_tree() should be called before we reparent the
preexisting tree on top of what we are adding.

Fixes: 3bd045cc ("separate copying and locking mount tree on cross-userns copies")
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>