In the Hyper-V drivers we need to allocate buffers shared between the
host and guest. This memory has been allocated with bus_dma, however
it doesn't use this correctly, e.g. it is missing calls to
bus_dmamap_sync. Along with this on arm64 we need this memory to be
mapped with the correct memory type that bus_dma may not use.
Switch to contigmalloc to allocate this memory as this will correctly
allocate cacheable memory.
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