udp: Add UFO to NETIF_F_SOFTWARE_GSO

This patch adds UFO to the list of GSO features with a software
fallback.  This allows UFO to be used even if the hardware does
not support it.

In particular, this allows us to test the UFO fallback, as it
has been reported to not work in some cases.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>