geneve: fix hlist corruption

It's not a good idea to add the same hlist_node to two different hash lists.
This leads to various hard to debug memory corruptions.

Fixes: 8ed66f0e ("geneve: implement support for IPv6-based tunnels")
Cc: John W. Linville <linville@tuxdriver.com>
Signed-off-by: Jiri Benc <jbenc@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>