netfilter: conntrack: tell compiler to not inline nf_ct_resolve_clash

At this time compiler inlines it, but this code will not be executed
under normal conditions.

Also, no inlining allows to use "nf_ct_resolve_clash%return" perf probe.

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