vxlan: Add support for UDP checksums (v4 sending, v6 zero csums)

Added VXLAN link configuration for sending UDP checksums, and allowing
TX and RX of UDP6 checksums.

Also, call common iptunnel_handle_offloads and added GSO support for
checksums.

Signed-off-by: Tom Herbert <therbert@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>