dm: use bio_list_merge_init

Use bio_list_merge_init instead of open coding bio_list_merge and
bio_list_init.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Mike Snitzer <snitzer@kernel.org>
Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Reviewed-by: Damien Le Moal <dlemoal@kernel.org>
Link: https://lore.kernel.org/r/20240328084147.2954434-4-hch@lst.de


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