kselftests: dmabuf-heaps: Add extra checking that allocated buffers are zeroed

Add a check to validate that buffers allocated from the heaps
are properly zeroed before being given to userland.

It is done by allocating a number of buffers, and filling them
with a nonzero pattern, then closing and reallocating more
buffers and checking that they are all properly zeroed.

This is helpful to validate any cached buffers are zeroed
before being given back out.

Cc: Shuah Khan <shuah@kernel.org>
Cc: Brian Starkey <brian.starkey@arm.com>
Cc: Sumit Semwal <sumit.semwal@linaro.org>
Cc: Laura Abbott <labbott@kernel.org>
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: linux-media@vger.kernel.org
Cc: dri-devel@lists.freedesktop.org
Cc: linux-kselftest@vger.kernel.org
Signed-off-by: John Stultz <john.stultz@linaro.org>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>