dmaengine: sa11x0: Annotate struct sa11x0_dma_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 sa11x0_dma_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: dmaengine@vger.kernel.org
Signed-off-by: Kees Cook <keescook@chromium.org>
Reviewed-by: "Gustavo A. R. Silva" <gustavoars@kernel.org>
Link: https://lore.kernel.org/r/20230817235859.49846-8-keescook@chromium.orgSigned-off-by: Vinod Koul <vkoul@kernel.org>

drivers/dma/sa11x0-dma.c

@@ -78,7 +78,7 @@ struct sa11x0_dma_desc {
 	bool			cyclic;
 
 	unsigned		sglen;
-	struct sa11x0_dma_sg	sg[];
+	struct sa11x0_dma_sg	sg[] __counted_by(sglen);
 };
 
 struct sa11x0_dma_phy;
@@ -558,6 +558,7 @@ static struct dma_async_tx_descriptor *sa11x0_dma_prep_slave_sg(
 		dev_dbg(chan->device->dev, "vchan %p: kzalloc failed\n", &c->vc);
 		return NULL;
 	}
+	txd->sglen = j;
 
 	j = 0;
 	for_each_sg(sg, sgent, sglen, i) {
@@ -593,7 +594,6 @@ static struct dma_async_tx_descriptor *sa11x0_dma_prep_slave_sg(
 
 	txd->ddar = c->ddar;
 	txd->size = size;
-	txd->sglen = j;
 
 	dev_dbg(chan->device->dev, "vchan %p: txd %p: size %zu nr %u\n",
 		&c->vc, &txd->vd, txd->size, txd->sglen);
@@ -628,6 +628,7 @@ static struct dma_async_tx_descriptor *sa11x0_dma_prep_dma_cyclic(
 		dev_dbg(chan->device->dev, "vchan %p: kzalloc failed\n", &c->vc);
 		return NULL;
 	}
+	txd->sglen = sglen;
 
 	for (i = k = 0; i < size / period; i++) {
 		size_t tlen, len = period;
@@ -653,7 +654,6 @@ static struct dma_async_tx_descriptor *sa11x0_dma_prep_dma_cyclic(
 
 	txd->ddar = c->ddar;
 	txd->size = size;
-	txd->sglen = sglen;
 	txd->cyclic = 1;
 	txd->period = sgperiod;