media: use struct_size() helper

Prefer struct_size() over open-coded versions of idiom:

sizeof(struct-with-flex-array) + sizeof(typeof-flex-array-elements) * count

where count is the max number of items the flexible array is supposed to
contain.
Signed-off-by: Yu Liao <liaoyu15@huawei.com>
Reviewed-by: Tomasz Figa <tfiga@chromium.org>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>

drivers/media/common/videobuf2/frame_vector.c

@@ -159,7 +159,7 @@ EXPORT_SYMBOL(frame_vector_to_pfns);
 struct frame_vector *frame_vector_create(unsigned int nr_frames)
 {
 	struct frame_vector *vec;
-	int size = sizeof(struct frame_vector) + sizeof(void *) * nr_frames;
+	int size = struct_size(vec, ptrs, nr_frames);
 
 	if (WARN_ON_ONCE(nr_frames == 0))
 		return NULL;