netfilter: flowtable: fix NULL pointer dereference in tunnel offload support

The tc ct action does not cache the route in the flowtable entry.

Fixes: 88bf6e41 ("netfilter: flowtable: add tunnel encap/decap action offload support")
Fixes: cfab6dbd ("netfilter: flowtable: add tunnel match offload support")
Signed-off-by: wenxu <wenxu@ucloud.cn>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>