bcache: Use struct_size() in kzalloc()

Make use of the struct_size() helper instead of an open-coded version
in order to avoid any potential type mistakes.

This code was detected with the help of Coccinelle and, audited and
fixed manually.
Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Signed-off-by: Coly Li <colyli@suse.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

drivers/md/bcache/writeback.c

@@ -459,10 +459,8 @@ static void read_dirty(struct cached_dev *dc)
 		for (i = 0; i < nk; i++) {
 			w = keys[i];
 
-			io = kzalloc(sizeof(struct dirty_io) +
-				     sizeof(struct bio_vec) *
-				     DIV_ROUND_UP(KEY_SIZE(&w->key),
-						  PAGE_SECTORS),
+			io = kzalloc(struct_size(io, bio.bi_inline_vecs,
+						DIV_ROUND_UP(KEY_SIZE(&w->key), PAGE_SECTORS)),
 				     GFP_KERNEL);
 			if (!io)
 				goto err;