netfilter: ctnetlink: memleak in filter initialization error path

Release the filter object in case of error.

Fixes: cb8aa9a3 ("netfilter: ctnetlink: add kernel side filtering for dump")
Reported-by:  <syzbot+38b8b548a851a01793c5@syzkaller.appspotmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>