netfilter: nf_conntrack: fix event flooding in GRE protocol tracker

GRE connections cause ctnetlink event flood because the ASSURED event
is set for every packet received.

Reported-by: Denys Fedoryshchenko <denys@visp.net.lb>
Tested-by: Denys Fedoryshchenko <denys@visp.net.lb>
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>