ANDROID: ion: flush cache before exporting non-cached buffers

The memory allocated from the page pool might be in the CPU cache, as the
zeroing can have been done through cache. e.g GFP_ZERO, or if the buffer
was used as cached memory previously. To map this memory as non-cached, we
need to flush the CPU cache first, to avoid any memory corruption when the
corresponding dirty cache line gets evicted.

A proper fix would be to flush the cache using the Linux DMA API. The
problem is that the buffer can be used without a device attached, which is
not covered since the introduction of:

  commit 1dccb598 ("arm64: simplify dma_get_ops")

This patch is then a dirty fix, calling directly cache flush function, to
cover the export of non-cached to an ION client. It ensures that the cache
line is actually cleaned before the memory is used as non-cached.

Change-Id: I9179c2f644a29ae71820c4ae63c880195f55e45d
Signed-off-by: Vincent Donnefort <vincent.donnefort@arm.com>