bcachefs: Fix bio alloc in check_extent_checksum()

if the buffer is virtually mapped it won't be a single bvec
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

fs/bcachefs/backpointers.c

@@ -470,7 +470,7 @@ static int check_extent_checksum(struct btree_trans *trans,
 		goto err;
 	}
 
-	bio = bio_alloc(ca->disk_sb.bdev, 1, REQ_OP_READ, GFP_KERNEL);
+	bio = bio_alloc(ca->disk_sb.bdev, buf_pages(data_buf, bytes), REQ_OP_READ, GFP_KERNEL);
 	bio->bi_iter.bi_sector = p.ptr.offset;
 	bch2_bio_map(bio, data_buf, bytes);
 	ret = submit_bio_wait(bio);