brd: use memcpy_{to,from]_page in brd_rw_bvec

Use the proper helpers to copy to/from potential highmem pages, which
do a local instead of atomic kmap underneath, and perform
flush_dcache_page where needed.  This also simplifies the code so much
that the separate read write helpers are not required any more.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Hannes Reinecke <hare@suse.de>
Reviewed-by: Yu Kuai <yukuai3@huawei.com>
Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Link: https://lore.kernel.org/r/20250428141014.2360063-6-hch@lst.de


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