fix loop checks in d_materialise_unique()

Both __d_unalias() and __d_materialise_dentry() need loop prevention.
Grab rename_lock in caller, check for loops there...

As a side benefit, we have dentry_lock_for_move() called only under
rename_lock, which seriously reduces deadlock potential of the
execrable "locking order" used for ->d_lock.

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