blk-mq: add a copyright notice to blk-mq-dma.c

blk-mq-dma.c was split from blk-merge.c which has no copyright notice,
but except for some boilerplate code and comments left from the old
version this is all my code, so add my copyright.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20250513071433.836797-2-hch@lst.de


Signed-off-by: Jens Axboe <axboe@kernel.dk>