dmaengine: stm32-mdma: Annotate struct stm32_mdma_desc 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_desc.
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/20230817235859.49846-13-keescook@chromium.orgSigned-off-by: Vinod Koul <vkoul@kernel.org>

drivers/dma/stm32-mdma.c

@@ -224,7 +224,7 @@ struct stm32_mdma_desc {
 	u32 ccr;
 	bool cyclic;
 	u32 count;
-	struct stm32_mdma_desc_node node[];
+	struct stm32_mdma_desc_node node[] __counted_by(count);
 };
 
 struct stm32_mdma_dma_config {
@@ -321,6 +321,7 @@ static struct stm32_mdma_desc *stm32_mdma_alloc_desc(
 	desc = kzalloc(struct_size(desc, node, count), GFP_NOWAIT);
 	if (!desc)
 		return NULL;
+	desc->count = count;
 
 	for (i = 0; i < count; i++) {
 		desc->node[i].hwdesc =
@@ -330,8 +331,6 @@ static struct stm32_mdma_desc *stm32_mdma_alloc_desc(
 			goto err;
 	}
 
-	desc->count = count;
-
 	return desc;
 
 err: