net/openvswitch: Set the ipv6 source tunnel key address attribute correctly

When dealing with ipv6 source tunnel key address attribute
(OVS_TUNNEL_KEY_ATTR_IPV6_SRC) we are wrongly setting the tunnel
dst ip, fix that.

Fixes: 6b26ba3a ('openvswitch: netlink attributes for IPv6 tunneling')
Signed-off-by: Or Gerlitz <ogerlitz@mellanox.com>
Reported-by: Paul Blakey <paulb@mellanox.com>
Acked-by: Jiri Benc <jbenc@redhat.com>
Acked-by: Joe Stringer <joe@ovn.org>
Signed-off-by: David S. Miller <davem@davemloft.net>