Add rbtree postorder iteration functions, runtime tests, and update zswap to use.

Postorder iteration yields all of a node's children prior to yielding the node
itself, and this particular implementation also avoids examining the leaf links
in a node after that node has been yielded.

In what I expect will be it's most common usage, postorder iteration allows the
deletion of every node in an rbtree without modifying the rbtree nodes (no
_requirement_ that they be nulled) while avoiding referencing child nodes after
they have been "deleted" (most commonly, freed).

I have only updated zswap to use this functionality at this point, but numerous
bits of code (most notably in the filesystem drivers) use a hand rolled
postorder iteration that NULLs child links as it traverses the tree. Each of
those instances could be replaced with this common implementation.

Cody P Schafer (5):
  rbtree: add postorder iteration functions.
  rbtree: add rbtree_postorder_for_each_entry_safe() helper.
  rbtree_test: add test for postorder iteration.
  rbtree: allow tests to run as builtin
  mm/zswap: use postorder iteration when destroying rbtree

 include/linux/rbtree.h | 21 +++++++++++++++++++++
 lib/Kconfig.debug      |  2 +-
 lib/rbtree.c           | 40 ++++++++++++++++++++++++++++++++++++++++
 lib/rbtree_test.c      | 12 ++++++++++++
 mm/zswap.c             | 15 ++-------------
 5 files changed, 76 insertions(+), 14 deletions(-)

-- 
1.8.3.4

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