dma-debug: report -EEXIST errors in add_dma_entry

Since, overlapping mappings are not supported by the DMA API we should
report an error if active_cacheline_insert returns -EEXIST.

Suggested-by: Dan Williams <dan.j.williams@intel.com>
Signed-off-by: Hamza Mahfooz <someguy@effective-light.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>