All the logic in iommu_dma_alloc that deals with page allocation from
the CMA or page allocators can be split into a self-contained helper,
and we can than map the result of that or the atomic pool allocation
with the iommu later.  This also allows reusing __iommu_dma_free to
tear down the allocations and MMU mappings when the IOMMU mapping
fails.

Based on a patch from Robin Murphy.
Signed-off-by: NChristoph Hellwig <hch@lst.de>
Signed-off-by: NJoerg Roedel <jroedel@suse.de>