blk-mq: move the DMA mapping code to a separate file

While working on the new DMA API I kept getting annoyed how it was placed
right in the middle of the bio splitting code in blk-merge.c.
Split it out into a separate file.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20250513071433.836797-1-hch@lst.de


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