netfilter: flowtable: fix IPv6 tunnel addr match

Previously the IPv6 addresses in the key were clobbered and the mask was
left unset.

I haven't tested this; I noticed it while skimming the code to
understand an unrelated issue.

Fixes: cfab6dbd ("netfilter: flowtable: add tunnel match offload support")
Cc: wenxu <wenxu@ucloud.cn>
Signed-off-by: Will Mortensen <willmo@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>