dmaengine: stm32-mdma: Use struct_size() in kzalloc()

Make use of the new struct_size() helper instead of the offsetof() idiom.
Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Link: https://lore.kernel.org/r/20201008141828.GA20325@embeddedorSigned-off-by: Vinod Koul <vkoul@kernel.org>

drivers/dma/stm32-mdma.c

@@ -339,7 +339,7 @@ static struct stm32_mdma_desc *stm32_mdma_alloc_desc(
 	struct stm32_mdma_desc *desc;
 	int i;
 
-	desc = kzalloc(offsetof(typeof(*desc), node[count]), GFP_NOWAIT);
+	desc = kzalloc(struct_size(desc, node, count), GFP_NOWAIT);
 	if (!desc)
 		return NULL;