netfilter: ctnetlink: do not erase error code with EINVAL

And be consistent in error management for both orig/reply filtering

Fixes: cb8aa9a3

 ("netfilter: ctnetlink: add kernel side filtering for dump")
Signed-off-by: Florent Fourcot <florent.fourcot@wifirst.fr>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>