s390/dasd: Use struct_size() helper

Make use of the struct_size() helper instead of an open-coded version
in order to avoid any potential type mistakes. Also, remove unnecessary
variable _datasize_.

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: Stefan Haberland <sth@linux.ibm.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

drivers/s390/block/dasd_diag.c

@@ -515,7 +515,7 @@ static struct dasd_ccw_req *dasd_diag_build_cp(struct dasd_device *memdev,
 	struct req_iterator iter;
 	struct bio_vec bv;
 	char *dst;
-	unsigned int count, datasize;
+	unsigned int count;
 	sector_t recid, first_rec, last_rec;
 	unsigned int blksize, off;
 	unsigned char rw_cmd;
@@ -543,10 +543,8 @@ static struct dasd_ccw_req *dasd_diag_build_cp(struct dasd_device *memdev,
 	if (count != last_rec - first_rec + 1)
 		return ERR_PTR(-EINVAL);
 	/* Build the request */
-	datasize = sizeof(struct dasd_diag_req) +
-		count*sizeof(struct dasd_diag_bio);
-	cqr = dasd_smalloc_request(DASD_DIAG_MAGIC, 0, datasize, memdev,
-				   blk_mq_rq_to_pdu(req));
+	cqr = dasd_smalloc_request(DASD_DIAG_MAGIC, 0, struct_size(dreq, bio, count),
+				   memdev, blk_mq_rq_to_pdu(req));
 	if (IS_ERR(cqr))
 		return cqr;