udpv6: Fix gso_size setting in ip6_ufo_append_data

- fix gso_size setting for ipv6 fragment to be a multiple of 8 bytes.

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>