bcache: use bvec_kmap_local in bio_csum

bcache: use bvec_kmap_local in bio_csum

Using local kmaps slightly reduces the chances to stray writes, and
the bvec interface cleans up the code a little bit.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Ira Weiny <ira.weiny@intel.com>
Link: https://lore.kernel.org/r/20220303111905.321089-8-hch@lst.de
Signed-off-by: Jens Axboe <axboe@kernel.dk>