udpv6: Fix HW checksum support for outgoing UFO packets

- add HW checksum support for outgoing large UDP/IPv6 packets destined for
  a UFO enabled device.

Signed-off-by: Sridhar Samudrala <sri@us.ibm.com>
Acked-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>