iommu/dma: Cleanup variable naming in iommu_dma_alloc

Most importantly clear up the size / iosize confusion.  Also rename addr
to cpu_addr to match the surrounding code and make the intention a little
more clear.

Signed-off-by: Robin Murphy <robin.murphy@arm.com>
[hch: split from a larger patch]
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Joerg Roedel <jroedel@suse.de>