drm/nouveau/pm: Annotate struct nvkm_perfdom 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 nvkm_perfdom.

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

Cc: Ben Skeggs <bskeggs@redhat.com>
Cc: Karol Herbst <kherbst@redhat.com>
Cc: Lyude Paul <lyude@redhat.com>
Cc: David Airlie <airlied@gmail.com>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: dri-devel@lists.freedesktop.org
Cc: nouveau@lists.freedesktop.org
Signed-off-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Lyude Paul <lyude@redhat.com>
Reviewed-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Signed-off-by: Christian König <christian.koenig@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230922173216.3823169-5-keescook@chromium.org

drivers/gpu/drm/nouveau/nvkm/engine/pm/priv.h

@@ -82,7 +82,7 @@ struct nvkm_perfdom {
 	u8  mode;
 	u32 clk;
 	u16 signal_nr;
-	struct nvkm_perfsig signal[];
+	struct nvkm_perfsig signal[] __counted_by(signal_nr);
 };
 
 struct nvkm_funcdom {