netfilter: conntrack: get rid of double sizeof

sizeof(sizeof()) is quite strange and does not seem to be what
is wanted here.

The issue is detected with the help of Coccinelle.

Fixes: 39215846 ("netfilter: conntrack: remove nlattr_size pointer from l4proto trackers")
Signed-off-by: zhong jiang <zhongjiang@huawei.com>
Acked-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>