media: usb: pwc-uncompress: Use struct_size() helper in pwc_decompress()

Make use of the struct_size() helper instead of an open-coded version,
in order to avoid any potential type mistakes or integer overflows that,
in the worst scenario, could lead to heap overflows.

Also, address the following sparse warnings:
drivers/media/usb/pwc/pwc-uncompress.c:44:44: warning: using sizeof on a flexible structure

Link: https://github.com/KSPP/linux/issues/174Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>

drivers/media/usb/pwc/pwc-uncompress.c

@@ -41,7 +41,7 @@ int pwc_decompress(struct pwc_device *pdev, struct pwc_frame_buf *fbuf)
 		memcpy(raw_frame->cmd, pdev->cmd_buf, 4);
 		memcpy(raw_frame+1, yuv, pdev->frame_size);
 		vb2_set_plane_payload(&fbuf->vb.vb2_buf, 0,
-			pdev->frame_size + sizeof(struct pwc_raw_frame));
+			struct_size(raw_frame, rawframe, pdev->frame_size));
 		return 0;
 	}