This function is for debug purpose only.  Currently it uses recursion for
tree traversal, checking an augmented value of each node to find out if it
is valid or not.

The recursion can corrupt the stack because the tree can be huge if
synthetic tests are applied.  To prevent it, navigate the tree from bottom
to upper levels using a regular list instead, because nodes are linked
among each other also.  It is faster and without recursion.
Signed-off-by: Uladzislau Rezki (Sony) <urezki@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Link: http://lkml.kernel.org/r/20200527205054.1696-2-urezki@gmail.comSigned-off-by: Linus Torvalds <torvalds@linux-foundation.org>