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 <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Jens Axboe <[email protected]>

Author: Christoph Hellwig

block/blk-mq-dma.c

@@ -1,4 +1,7 @@
 // SPDX-License-Identifier: GPL-2.0-only
+/*
+ * Copyright (C) 2025 Christoph Hellwig
+ */
 #include "blk.h"
 
 struct phys_vec {