netfilter: nfnetlink_queue: un-break NF_REPEAT

Only override userspace verdict if the ct hook returns something
other than ACCEPT.

Else, this replaces NF_REPEAT (run all hooks again) with NF_ACCEPT
(move to next hook).

Fixes: 6291b3a6

 ("netfilter: conntrack: convert nf_conntrack_update to netfilter verdicts")
Reported-by:  <l.6diay@passmail.com>
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>