geneve: testing the wrong variable in geneve6_build_skb()

We intended to test "err" and not "skb".

Fixes: aed069df

 ('ip_tunnel_core: iptunnel_handle_offloads returns int and doesn't free skb')
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Alexander Duyck <aduyck@mirantis.com>
Signed-off-by: David S. Miller <davem@davemloft.net>