brd: use bvec_kmap_local in brd_do_bvec

Use the proper helper to kmap a bvec in brd_do_bvec instead of directly
accessing the bvec fields and use the deprecated kmap_atomic API.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Yu Kuai <yukuai3@huawei.com>
Reviewed-by: Hannes Reinecke <hare@suse.de>
Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Link: https://lore.kernel.org/r/20250428141014.2360063-4-hch@lst.de


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