netfilter: conntrack: allow insertion clash of gre protocol

NVGRE tunnel is used in the VM-to-VM communications. The VM packets
are encapsulated in NVGRE and sent from the host. For NVGRE
there are two tuples(outer sip and outer dip) in the host conntrack item.
Insertion clashes are more likely to happen if the concurrent connections
are sent from the VM.

Signed-off-by: Faicker Mo <faicker.mo@ucloud.cn>
Signed-off-by: Florian Westphal <fw@strlen.de>