media: allegro: Annotate struct mcu_msg_push_buffers_internal 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 mcu_msg_push_buffers_internal.

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

Cc: Michael Tretter <[e-mail]>
Cc: Pengutronix Kernel Team <[e-mail]>
Signed-off-by: Kees Cook <[e-mail]>
Reviewed-by: Gustavo A. R. Silva <[e-mail]>
Signed-off-by: Hans Verkuil <[e-mail]>

drivers/media/platform/allegro-dvt/allegro-mail.h

···
 	struct mcu_msg_header header;
 	u32 channel_id;
 	size_t num_buffers;
-	struct mcu_msg_push_buffers_internal_buffer buffer[];
+	struct mcu_msg_push_buffers_internal_buffer buffer[] __counted_by(num_buffers);
 };
 
 struct mcu_msg_put_stream_buffer {