Instead of having this mysterious private_data in each radix_tree_node,
store a pointer to the root, which can be useful for debugging.  This also
relieves the mm code from the duty of updating it.
Acked-by: NJohannes Weiner <hannes@cmpxchg.org>
Signed-off-by: NMatthew Wilcox <mawilcox@microsoft.com>