block: take destination bvec offsets into account in bio_copy_data_iter

[ Upstream commit 403d50341c ]

Appartly bcache can copy into bios that do not just contain fresh
pages but can have offsets into the bio_vecs.  Restore support for tht
in bio_copy_data_iter.

Fixes: f8b679a070 ("block: rewrite bio_copy_data_iter to use bvec_kmap_local and memcpy_to_bvec")
Signed-off-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20220524143919.1155501-1-hch@lst.de
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Sasha Levin <sashal@kernel.org>

block/bio.c

@@ -1289,10 +1289,12 @@ void bio_copy_data_iter(struct bio *dst, struct bvec_iter *dst_iter,
 		struct bio_vec src_bv = bio_iter_iovec(src, *src_iter);
 		struct bio_vec dst_bv = bio_iter_iovec(dst, *dst_iter);
 		unsigned int bytes = min(src_bv.bv_len, dst_bv.bv_len);
-		void *src_buf;
+		void *src_buf = bvec_kmap_local(&src_bv);
+		void *dst_buf = bvec_kmap_local(&dst_bv);
 
-		src_buf = bvec_kmap_local(&src_bv);
-		memcpy_to_bvec(&dst_bv, src_buf);
+		memcpy(dst_buf, src_buf, bytes);
+
+		kunmap_local(dst_buf);
 		kunmap_local(src_buf);
 
 		bio_advance_iter_single(src, src_iter, bytes);