dma-debug: small dma_debug_entry's comment and variable name updates

Small update on dma_debug_entry's struct commentary and also standardize
the usage of 'dma_addr' variable name from debug_dma_map_page() on
debug_dma_unmap_page(), and similarly on debug_dma_free_coherent()

Signed-off-by: Desnes Nunes <desnesn@redhat.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>