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

drivers/md/bcache/request.c

@@ -44,10 +44,10 @@ static void bio_csum(struct bio *bio, struct bkey *k)
 	uint64_t csum = 0;
 
 	bio_for_each_segment(bv, bio, iter) {
-		void *d = kmap(bv.bv_page) + bv.bv_offset;
+		void *d = bvec_kmap_local(&bv);
 
 		csum = crc64_be(csum, d, bv.bv_len);
-		kunmap(bv.bv_page);
+		kunmap_local(d);
 	}
 
 	k->ptr[KEY_PTRS(k)] = csum & (~0ULL >> 1);