netfilter: conntrack: remove two args from resolve_clash

ctinfo is whats taken from the skb, i.e.
ct = nf_ct_get(skb, &ctinfo).

We do not pass 'ct' and instead re-fetch it from the skb.
Just do the same for both netns and ctinfo.

Also add a comment on what clash resolution is supposed to do.
While at it, one indent level can be removed.

Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>