dmaengine: usb-dmac: Annotate struct usb_dmac_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 usb_dmac_desc.

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

Cc: Vinod Koul <vkoul@kernel.org>
Cc: Yu Kuai <yukuai3@huawei.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Jordy Zomer <jordy@pwning.systems>
Cc: Jernej Skrabec <jernej.skrabec@gmail.com>
Cc: Tudor Ambarus <tudor.ambarus@linaro.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/20230817235859.49846-21-keescook@chromium.orgSigned-off-by: Vinod Koul <vkoul@kernel.org>

drivers/dma/sh/usb-dmac.c

@@ -57,7 +57,7 @@ struct usb_dmac_desc {
 	u32 residue;
 	struct list_head node;
 	dma_cookie_t done_cookie;
-	struct usb_dmac_sg sg[];
+	struct usb_dmac_sg sg[] __counted_by(sg_allocated_len);
 };
 
 #define to_usb_dmac_desc(vd)	container_of(vd, struct usb_dmac_desc, vd)