dm crypt: use struct_size() when allocating encryption context

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-crypt.c

@@ -2699,7 +2699,7 @@ static int crypt_ctr(struct dm_target *ti, unsigned int argc, char **argv)
 		return -EINVAL;
 	}
 
-	cc = kzalloc(sizeof(*cc) + key_size * sizeof(u8), GFP_KERNEL);
+	cc = kzalloc(struct_size(cc, key, key_size), GFP_KERNEL);
 	if (!cc) {
 		ti->error = "Cannot allocate encryption context";
 		return -ENOMEM;