rsxx: Use struct_size() in vmalloc()

Make use of the struct_size() helper instead of an open-coded version,
in order to avoid any potential type mistakes or integer overflows
that, in the worst scenario, could lead to heap overflows.

This code was detected with the help of Coccinelle and, audited and
fixed manually.
Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Link: https://lore.kernel.org/r/20210513203730.GA212128@embeddedorSigned-off-by: Jens Axboe <axboe@kernel.dk>

drivers/block/rsxx/dma.c

@@ -74,9 +74,6 @@ struct dma_tracker {
 	struct rsxx_dma	*dma;
 };
 
-#define DMA_TRACKER_LIST_SIZE8 (sizeof(struct dma_tracker_list) + \
-		(sizeof(struct dma_tracker) * RSXX_MAX_OUTSTANDING_CMDS))
-
 struct dma_tracker_list {
 	spinlock_t		lock;
 	int			head;
@@ -808,7 +805,8 @@ static int rsxx_dma_ctrl_init(struct pci_dev *dev,
 
 	memset(&ctrl->stats, 0, sizeof(ctrl->stats));
 
-	ctrl->trackers = vmalloc(DMA_TRACKER_LIST_SIZE8);
+	ctrl->trackers = vmalloc(struct_size(ctrl->trackers, list,
+					     RSXX_MAX_OUTSTANDING_CMDS));
 	if (!ctrl->trackers)
 		return -ENOMEM;