net/sched: cls_u32: Use struct_size() helper

Make use of the struct_size() helper, in multiple places, instead
of an open-coded version in order to avoid any potential type
mistakes and protect against potential integer overflows.

Also, remove unnecessary object identifier size.

Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>