dmaengine: ti: omap-dma: Annotate struct omap_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 omap_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: Peter Ujfalusi <peter.ujfalusi@gmail.com>
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/20230817235859.49846-18-keescook@chromium.orgSigned-off-by: Vinod Koul <vkoul@kernel.org>

drivers/dma/ti/omap-dma.c

@@ -124,7 +124,7 @@ struct omap_desc {
 	uint32_t csdp;		/* CSDP value */
 
 	unsigned sglen;
-	struct omap_sg sg[];
+	struct omap_sg sg[] __counted_by(sglen);
 };
 
 enum {
@@ -1005,6 +1005,7 @@ static struct dma_async_tx_descriptor *omap_dma_prep_slave_sg(
 	d = kzalloc(struct_size(d, sg, sglen), GFP_ATOMIC);
 	if (!d)
 		return NULL;
+	d->sglen = sglen;
 
 	d->dir = dir;
 	d->dev_addr = dev_addr;
@@ -1120,8 +1121,6 @@ static struct dma_async_tx_descriptor *omap_dma_prep_slave_sg(
 		}
 	}
 
-	d->sglen = sglen;
-
 	/* Release the dma_pool entries if one allocation failed */
 	if (ll_failed) {
 		for (i = 0; i < d->sglen; i++) {