dmaengine: tegra: Annotate struct tegra_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 tegra_dma_desc.

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

Cc: Vinod Koul <vkoul@kernel.org>
Cc: Laxman Dewangan <ldewangan@nvidia.com>
Cc: Jon Hunter <jonathanh@nvidia.com>
Cc: Thierry Reding <thierry.reding@gmail.com>
Cc: dmaengine@vger.kernel.org
Cc: linux-tegra@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/20230817235859.49846-15-keescook@chromium.orgSigned-off-by: Vinod Koul <vkoul@kernel.org>

drivers/dma/tegra186-gpc-dma.c

@@ -221,7 +221,7 @@ struct tegra_dma_desc {
 	unsigned int sg_count;
 	struct virt_dma_desc vd;
 	struct tegra_dma_channel *tdc;
-	struct tegra_dma_sg_req sg_req[];
+	struct tegra_dma_sg_req sg_req[] __counted_by(sg_count);
 };
 
 /*