geneve: UDP checksum configuration via netlink

geneve: UDP checksum configuration via netlink

BugLink: http://bugs.launchpad.net/bugs/1536474
Add support to enable and disable UDP checksums via netlink. This is
similar to how VXLAN and GUE allow this. This includes support for
enabling the UDP zero checksum (for both TX and RX).

Signed-off-by: Tom Herbert <tom@herbertland.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
(back ported from commit abe492b4f50c3ae2ebcfaa2f5c16176aebaa1c68)
Signed-off-by: Tim Gardner <tim.gardner@canonical.com>