geneve: only configure or fill UDP_ZERO_CSUM6_RX/TX info when CONFIG_IPV6

Stefano pointed that configure or show UDP_ZERO_CSUM6_RX/TX info doesn't
make sense if we haven't enabled CONFIG_IPV6. Fix it by adding
if IS_ENABLED(CONFIG_IPV6) check.

Fixes: abe492b4 ("geneve: UDP checksum configuration via netlink")
Fixes: fd7eafd0

 ("geneve: fix fill_info when link down")
Signed-off-by: Hangbin Liu <liuhangbin@gmail.com>
Reviewed-by: Stefano Brivio <sbrivio@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>