dmaengine: stm32-mdma: Annotate struct stm32_mdma_device with __counted_by

Prepare for the coming implementation by GCC and Clang of the __counted_by
attribute. Flexible array members annotated with __counted_by can have
their accesses bounds-checked at run-time checking via CONFIG_UBSAN_BOUNDS
(for array indexing) and CONFIG_FORTIFY_SOURCE (for strcpy/memcpy-family
functions).

As found with Coccinelle[1], add __counted_by for struct stm32_mdma_device.
Additionally, since the element count member must be set before accessing
the annotated flexible array member, move its initialization earlier.

[1] https://github.com/kees/kernel-tools/blob/trunk/coccinelle/examples/counted_by.cocci

Cc: Vinod Koul <vkoul@kernel.org>
Cc: Maxime Coquelin <mcoquelin.stm32@gmail.com>
Cc: Alexandre Torgue <alexandre.torgue@foss.st.com>
Cc: dmaengine@vger.kernel.org
Cc: linux-stm32@st-md-mailman.stormreply.com
Cc: linux-arm-kernel@lists.infradead.org
Signed-off-by: Kees Cook <keescook@chromium.org>
Reviewed-by: "Gustavo A. R. Silva" <gustavoars@kernel.org>
Link: https://lore.kernel.org/r/20230817235859.49846-14-keescook@chromium.orgSigned-off-by: Vinod Koul <vkoul@kernel.org>

drivers/dma/stm32-mdma.c

@@ -256,7 +256,7 @@ struct stm32_mdma_device {
 	u32 nr_ahb_addr_masks;
 	u32 chan_reserved;
 	struct stm32_mdma_chan chan[STM32_MDMA_MAX_CHANNELS];
-	u32 ahb_addr_masks[];
+	u32 ahb_addr_masks[] __counted_by(nr_ahb_addr_masks);
 };
 
 static struct stm32_mdma_device *stm32_mdma_get_dev(
@@ -1611,13 +1611,13 @@ static int stm32_mdma_probe(struct platform_device *pdev)
 			      GFP_KERNEL);
 	if (!dmadev)
 		return -ENOMEM;
+	dmadev->nr_ahb_addr_masks = count;
 
 	dmadev->nr_channels = nr_channels;
 	dmadev->nr_requests = nr_requests;
 	device_property_read_u32_array(&pdev->dev, "st,ahb-addr-masks",
 				       dmadev->ahb_addr_masks,
 				       count);
-	dmadev->nr_ahb_addr_masks = count;
 
 	dmadev->base = devm_platform_ioremap_resource(pdev, 0);
 	if (IS_ERR(dmadev->base))