netfilter: conntrack: gre: don't set assured flag for clash entries

Now that conntrack core is allowd to insert clashing entries, make sure
GRE won't set assured flag on NAT_CLASH entries, just like UDP.

Doing so prevents early_drop logic for these entries.

Fixes: d671fd82 ("netfilter: conntrack: allow insertion clash of gre protocol")
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>