[NETFILTER] ctnetlink: return -EINVAL if size is wrong

Return -EINVAL if the size isn't OK instead of -EPERM.

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: Harald Welte <laforge@netfilter.org>
Signed-off-by: David S. Miller <davem@davemloft.net>