media: v4l2-event: Annotate struct v4l2_subscribed_event 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 v4l2_subscribed_event.
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: lijian <[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/v4l2-core/v4l2-event.c

···
 	sev = kvzalloc(struct_size(sev, events, elems), GFP_KERNEL);
 	if (!sev)
 		return -ENOMEM;
+	sev->elems = elems;
 	for (i = 0; i < elems; i++)
 		sev->events[i].sev = sev;
 	sev->type = sub->type;
···
 	sev->flags = sub->flags;
 	sev->fh = fh;
 	sev->ops = ops;
-	sev->elems = elems;
 
 	mutex_lock(&fh->subscribe_lock);
 

include/media/v4l2-event.h

···
 	unsigned int		elems;
 	unsigned int		first;
 	unsigned int		in_use;
-	struct v4l2_kevent	events[];
+	struct v4l2_kevent	events[] __counted_by(elems);
 };
 
 /**