dmaengine: stm32-dma: Use struct_size() helper

One of the more common cases of allocation size calculations is finding
the size of a structure that has a zero-sized array at the end, along
with memory for some number of elements for that array. For example:

struct stm32_dma_desc {
	...
        struct stm32_dma_sg_req sg_req[];
};

Make use of the struct_size() helper instead of an open-coded version
in order to avoid any potential type mistakes.

So, replace the following function:

static struct stm32_dma_desc *stm32_dma_alloc_desc(u32 num_sgs)
{
       return kzalloc(sizeof(struct stm32_dma_desc) +
                      sizeof(struct stm32_dma_sg_req) * num_sgs, GFP_NOWAIT);
}

with:

kzalloc(struct_size(desc, sg_req, num_sgs), GFP_NOWAIT)

This code was detected with the help of Coccinelle.
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Link: https://lore.kernel.org/r/20190830161423.GA3483@embeddedorSigned-off-by: Vinod Koul <vkoul@kernel.org>

drivers/dma/stm32-dma.c

@@ -243,12 +243,6 @@ static void stm32_dma_write(struct stm32_dma_device *dmadev, u32 reg, u32 val)
 	writel_relaxed(val, dmadev->base + reg);
 }
 
-static struct stm32_dma_desc *stm32_dma_alloc_desc(u32 num_sgs)
-{
-	return kzalloc(sizeof(struct stm32_dma_desc) +
-		       sizeof(struct stm32_dma_sg_req) * num_sgs, GFP_NOWAIT);
-}
-
 static int stm32_dma_get_width(struct stm32_dma_chan *chan,
 			       enum dma_slave_buswidth width)
 {
@@ -853,7 +847,7 @@ static struct dma_async_tx_descriptor *stm32_dma_prep_slave_sg(
 		return NULL;
 	}
 
-	desc = stm32_dma_alloc_desc(sg_len);
+	desc = kzalloc(struct_size(desc, sg_req, sg_len), GFP_NOWAIT);
 	if (!desc)
 		return NULL;
 
@@ -954,7 +948,7 @@ static struct dma_async_tx_descriptor *stm32_dma_prep_dma_cyclic(
 
 	num_periods = buf_len / period_len;
 
-	desc = stm32_dma_alloc_desc(num_periods);
+	desc = kzalloc(struct_size(desc, sg_req, num_periods), GFP_NOWAIT);
 	if (!desc)
 		return NULL;
 
@@ -989,7 +983,7 @@ static struct dma_async_tx_descriptor *stm32_dma_prep_dma_memcpy(
 	int i;
 
 	num_sgs = DIV_ROUND_UP(len, STM32_DMA_ALIGNED_MAX_DATA_ITEMS);
-	desc = stm32_dma_alloc_desc(num_sgs);
+	desc = kzalloc(struct_size(desc, sg_req, num_sgs), GFP_NOWAIT);
 	if (!desc)
 		return NULL;