dm log writes: use struct_size() to calculate size of pending_block

Use struct_size() to avoid open-coded equivalent that is prone to a type
mistake.
Signed-off-by: Zhengyuan Liu <liuzhengyuan@kylinos.cn>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>

drivers/md/dm-log-writes.c

@@ -699,7 +699,7 @@ static int log_writes_map(struct dm_target *ti, struct bio *bio)
 	if (discard_bio)
 		alloc_size = sizeof(struct pending_block);
 	else
-		alloc_size = sizeof(struct pending_block) + sizeof(struct bio_vec) * bio_segments(bio);
+		alloc_size = struct_size(block, vecs, bio_segments(bio));
 
 	block = kzalloc(alloc_size, GFP_NOIO);
 	if (!block) {