iommu/arm-smmu: fix leak in arm_smmu_flush_pgtable

We're currently mapping a page in arm_smmu_flush_pgtable without ever
unmapping it.  Fix this by calling dma_unmap_page on the returned dma
address.  Since the only reason we're calling dma_map_page is to make
sure it actually gets flushed out to RAM, we can just call
dma_unmap_page immediately following the map.

Without this, eventually swiotlb runs out of memory and starts printing
things like:

    arm-smmu d00000.arm,smmu: swiotlb buffer is full (sz: 128 bytes)

Change-Id: I69421de6e2189a16cd88f225e2698cd88d669dff
Signed-off-by: Mitchel Humpherys <mitchelh@codeaurora.org>