[$] Destaging ION

The Android system has shipped a couple of allocators for DMA buffersover the years; first came PMEM, then itsreplacement ION. The ION allocator hasbeen in use since around 2012, but it remains stuck in the kernel's stagingtree. The work to add ION to the mainline started in 2013;at that time, the allocator had multiple issues that made inclusionimpossible. Recently, John Stultz posteda patch set introducing DMA-BUF heaps, an evolution of ION, that isdesigned to do exactly that - get the Android DMA-buffer allocator tothe mainline Linux kernel.