radix-tree: fix private list warnings

The newly introduced warning in radix_tree_free_nodes() was testing the
wrong variable; it should have been 'old' instead of 'node'.

Fixes: ea07b862

 ("mm: workingset: fix use-after-free in shadow node shrinker")
Link: http://lkml.kernel.org/r/20170118163746.GA32495@cmpxchg.org
Signed-off-by: Matthew Wilcox <mawilcox@microsoft.com>
Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>