ANDROID: dma-buf: heaps: Add deferred-free-helper library code

This patch provides infrastructure for deferring buffer frees.

This is a feature ION provided which when used with some form
of a page pool, provides a nice performance boost in an
allocation microbenchmark. The reason it helps is it allows the
page-zeroing to be done out of the normal allocation/free path,
and pushed off to a kthread.

As not all heaps will find this useful, its implemented as
a optional helper library that heaps can utilize.

Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: Sumit Semwal <sumit.semwal@linaro.org>
Cc: Liam Mark <lmark@codeaurora.org>
Cc: Chris Goldsworthy <cgoldswo@codeaurora.org>
Cc: Laura Abbott <labbott@kernel.org>
Cc: Brian Starkey <Brian.Starkey@arm.com>
Cc: Hridya Valsaraju <hridya@google.com>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Sandeep Patil <sspatil@google.com>
Cc: Daniel Mentz <danielmentz@google.com>
Cc: Ørjan Eide <orjan.eide@arm.com>
Cc: Robin Murphy <robin.murphy@arm.com>
Cc: Ezequiel Garcia <ezequiel@collabora.com>
Cc: Simon Ser <contact@emersion.fr>
Cc: James Jones <jajones@nvidia.com>
Cc: linux-media@vger.kernel.org
Cc: dri-devel@lists.freedesktop.org
Signed-off-by: John Stultz <john.stultz@linaro.org>
Bug: 168742043
Change-Id: Ibdc6a6f44a6d7583817bdc808f8d4fb836b0210f
---
v2:
* Fix sleep in atomic issue from using a mutex, by switching
  to a spinlock as Reported-by: kernel test robot <oliver.sang@intel.com>
* Cleanup API to use a reason enum for clarity and add some documentation
  comments as suggested by Suren Baghdasaryan.
v3:
* Minor tweaks so it can be built as a module
* A few small fixups suggested by Daniel Mentz
v4:
* Tweak from Daniel Mentz to make sure the shrinker
  count/freed values are tracked in pages not bytes
v5:
* Fix up page count tracking as suggested by Suren Baghdasaryan