ibmveth: Fix issue with DMA mapping failure

descs[].fields.address is 32bit which truncates any dma mapping
errors so dma_mapping_error() fails to catch it.

Use a dma_addr_t to do the comparison. With this patch I was able
to transfer many gigabytes of data with IOMMU fault injection set
at 10% probability.

Signed-off-by: Anton Blanchard <anton@samba.org>
Cc: <stable@kernel.org> # v2.6.37+
Signed-off-by: David S. Miller <davem@davemloft.net>